[llvm] [SPIR-V] Fix OpDecorate emission after vreg def. (PR #114426)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

There are 2 broken tests, also broken on main:
 - CodeGen/SPIRV/transcoding/RelationalOperators.ll
 - CodeGen/SPIRV/uitofp-with-bool.ll
Those are unrelated to this PR.

https://github.com/llvm/llvm-project/pull/114426