AMDGPU: Extract full 64-bit subregister and use subregs

Instead of extracting both 32-bit components from the 128-bit
register. This produces fewer copies and is easier for
the copy peephole optimizer to understand and see the actual uses
as extracts from a reg_sequence.

This avoids needing to handle subregister composing in the
PeepholeOptimizer's ValueTracker for this case.

llvm-svn: 247162