Simplify and compact switch ZIR, and resolve union payload captures with PTR

[mlugg]

While looking to implement #2473 (Coming Soon ™️), I found that the existing representation of switch expressions in ZIR was incompatible with that proposal. Further inspection led me to conclude that the existing representation in ZIR is unnecessarily large, and also uses far more tags than it needs to.

All but the last commit of this PR are dedicated to simplifying this ZIR. The 8 tags used in master for switch expressions have been reduced down to 2, and the size in memory of the ZIR for switch expressions is decreased across the board (stats below).

The new encoding has two instructions: switch_block and switch_block_ref, where the latter is for passing the operand by reference. Some bits in the trailing data are repurposed to provide information about captures. The main capture is referenced through the switch block instruction itself (since this otherwise can't be referenced inside a prong), and inline tag captures, if present, are referenced through a dummy instruction, like how errdefer captures work.

The final commit of this PR implements #2812. See its commit message for a bit more information on the strategy there.

ZIR stats (prior to proposal implementation, which will not affect the ZIR):

File	Old	New	Change
test/behavior/inline_switch.zig	20.26K	19.39K	-4.3%
test/behavior/switch.zig	80.58K	77.20K	-4.2%
src/Sema.zig	4.57M	4.50M	-1.4%
src/value.zig	795.33K	768.85K	-3.3%

[andrewrk]

perf data point for self-hosted compiler (2% slower):

Benchmark 1: before/bin/zig build-exe --stack 33554432 /home/andy/dev/zig/src/main.zig --cache-dir /home/andy/dev/zig/zig-cache --global-cache-dir /home/andy/.cache/zig --name zig --mod build_options::/home/andy/dev/zig/zig-cache/c/58a1564dccbec7b9340b137525e685a4/options.zig --deps build_options --zig-lib-dir /home/andy/dev/zig/lib -fno-emit-bin
  Time (mean ± σ):      7.018 s ±  0.083 s    [User: 6.911 s, System: 0.185 s]
  Range (min … max):    6.953 s …  7.112 s    3 runs
 
Benchmark 2: after/bin/zig build-exe --stack 33554432 /home/andy/dev/zig/src/main.zig --cache-dir /home/andy/dev/zig/zig-cache --global-cache-dir /home/andy/.cache/zig --name zig --mod build_options::/home/andy/dev/zig/zig-cache/c/58a1564dccbec7b9340b137525e685a4/options.zig --deps build_options --zig-lib-dir /home/andy/dev/zig/lib -fno-emit-bin
  Time (mean ± σ):      7.136 s ±  0.030 s    [User: 7.043 s, System: 0.168 s]
  Range (min … max):    7.105 s …  7.164 s    3 runs
 
Summary
  'before/bin/zig build-exe --stack 33554432 /home/andy/dev/zig/src/main.zig --cache-dir /home/andy/dev/zig/zig-cache --global-cache-dir /home/andy/.cache/zig --name zig --mod build_options::/home/andy/dev/zig/zig-cache/c/58a1564dccbec7b9340b137525e685a4/options.zig --deps build_options --zig-lib-dir /home/andy/dev/zig/lib -fno-emit-bin' ran
    1.02 ± 0.01 times faster than 'after/bin/zig build-exe --stack 33554432 /home/andy/dev/zig/src/main.zig --cache-dir /home/andy/dev/zig/zig-cache --global-cache-dir /home/andy/.cache/zig --name zig --mod build_options::/home/andy/dev/zig/zig-cache/c/58a1564dccbec7b9340b137525e685a4/options.zig --deps build_options --zig-lib-dir /home/andy/dev/zig/lib -fno-emit-bin'