Implement the getRegForInlineAsmConstraint method for PPC. With recent

Implement the getRegForInlineAsmConstraint method for PPC.  With recent
sdisel changes, this eliminates a ton of copies around common inline asms.
For example:

int test2(int Y, int X) {
  asm("foo %0, %1" : "=r"(X): "r"(X));
  return X;
}

now compiles to:

_test2:
        foo r3, r4
        blr

instead of:

_test2:
        mr r2, r4
        foo r2, r2
        mr r3, r2
        blr

GCC produces:

_test2:
        foo r4, r4
        mr r3,r4
        blr

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31367 91177308-0d34-0410-b5e6-96231b3b80d8