[llvm] [SPIR-V] Do instruction selection for G_BITCAST on an earlier stage (PR #114216)

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy wrote:

@Keenuts This should fix all those `TODO(pull/110270): verifier, fix G_BITCAST error "bitcast must change type"` in https://github.com/llvm/llvm-project/pull/111026

https://github.com/llvm/llvm-project/pull/114216