AMDGPU: refactor phi lowering from SILowerI1Copies (NFCI) (#75349)

Make abstract class PhiLoweringHelper and expose it for use in
GlobalISel path.
SILowerI1Copies implements PhiLoweringHelper as Vreg1LoweringHelper and
it is equivalent to SILowerI1Copies.
Notable change that createLaneMaskReg now clones attributes from
register that has lane mask attributes instead of creating register with
lane mask register class. This is because lane masks have
different(more) attributes in GlobalISel.

patch 2 from: https://github.com/llvm/llvm-project/pull/73337