The PPC MFCR instruction implicitly uses all 8 of the CR

The PPC MFCR instruction implicitly uses all 8 of the CR
registers.  Currently it is not so marked, which leads to
VCMPEQ instructions that feed into it getting deleted.
If it is so marked, local RA complains about this sequence:
 vreg = MCRF  CR0
 MFCR  <kill of whatever preg got assigned to vreg>
All current uses of this instruction are only interested in
one of the 8 CR registers, so redefine MFCR to be a normal
unary instruction with a CR input (which is emitted only as
a comment).  That avoids all problems.  7739628.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104238 91177308-0d34-0410-b5e6-96231b3b80d8