Fix a miscompilation of:

Fix a miscompilation of:

long long foo(long long X) {
  return (long long)(signed char)(int)X;
}

Instead of:

_foo:
        extsb r2, r4
        srawi r3, r4, 31
        mr r4, r2
        blr

we now produce:

_foo:
        extsb r4, r4
        srawi r3, r4, 31
        blr

This fixes a miscompilation in ConstantFolding.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30768 91177308-0d34-0410-b5e6-96231b3b80d8