[llvm] [AMDGPU] SIFoldOperands: update BUNDLE header implicit use when folding (PR #201872)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

moved inside else else block, this should not be valid with physical registers.
updated comment and the comment in mir test

https://github.com/llvm/llvm-project/pull/201872