[spv-out] ZeroValue expressions produce too many OpConstantNull instructions

[jimblandy]

#2332 introduces Expression::ZeroValue, which the SPIR-V backend uses to produce OpConstantNull instructions. However, it doesn't check to see if it's already got an an appropriate null constant for the given type, resulting in code like this:

%274 = OpConstantNull %10
%275 = OpConstantNull %10
%276 = OpConstantNull %13
%277 = OpConstantNull %10
%278 = OpConstantNull %8
%279 = OpConstantNull %10
%280 = OpConstantNull %8
%281 = OpConstantNull %10
%282 = OpConstantNull %8
%283 = OpConstantNull %13
%284 = OpConstantNull %8

Only three of these are necessary.

This example appears is in the snapshot output tests/out/spv/bounds-check-image-rzsw.spvasm.

[teoxoy]

I think we cache all other constants besides the null variant. This shouldn't be too hard to do considering we have the system in place for it.

[evahop]

I think this is something I didn't catch while doing the composites. I can tackle this