AMDGPU: Fix verifier error in SIFoldOperands

There may be other use operands that also need their kill flags cleared.

This happens in a few tests when SIFoldOperands is moved after
PeepholeOptimizer.

PeepholeOptimizer rewrites cases that look like:
%vreg0 = ...
%vreg1 = COPY %vreg0
use %vreg1<kill>
%vreg2 = COPY %vreg0
use %vreg2<kill>

to use the earlier source to
%vreg0 = ...
use %vreg0
use %vreg0

Currently SIFoldOperands sees the copied registers, so there is
only one use. So far I haven't managed to come up with a test
that currently has multiple uses of a foldable VGPR -> VGPR copy.

llvm-svn: 250960