[llvm] [SPIRV] Audit `select` Result in SPIRVInstructionSelector (PR #115193)

[Sarah Spall via llvm-commits]

https://github.com/spall commented:

Mostly I think you should stop pre-defining Result to be true, and just define it the first place you do &=.

https://github.com/llvm/llvm-project/pull/115193