HLSL-IR: implement FirstIndexOffset #4740

Job	Run time
Go Build and Test	31s
CMake-windows-latest-Debug-msvc	14m 14s
CMake-windows-latest-Release-msvc	16m 55s
CMake-ubuntu-latest-Debug-gcc	18m 48s
CMake-ubuntu-latest-Release-gcc	20m 31s
CMake-macos-latest-Debug-clang	20m 10s
CMake-macos-latest-Release-clang	28m 21s
CMake-macos-13-Debug-clang	51m 59s
CMake-macos-13-Release-clang	1h 5m 53s
	3h 57m 22s

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