AMDGPU select splitting combines need to ensure condition is not undef

[arsenm]

AMDGPU has various transformations that break up 64-bit operations into 32-bit pieces, many of which appear to be unsafe with undef. e.g. select splitting: https://alive2.llvm.org/ce/z/b5gN6V

This needs to insert a freeze.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Matt Arsenault (arsenm)

AMDGPU has various transformations that break up 64-bit operations into 32-bit pieces, many of which appear to be unsafe with undef. e.g. select splitting: https://alive2.llvm.org/ce/z/b5gN6V

This needs to insert a freeze.