SIGN_EXTEND_INREG does not demand its top bits. Give SimplifyDemandedBits

SIGN_EXTEND_INREG does not demand its top bits.  Give SimplifyDemandedBits
a chance to hack on it.  This compiles:

int baz(long long a) { return (short)(((int)(a >>24)) >> 9); }

into:
_baz:
        slwi r2, r3, 8
        srwi r2, r2, 9
        extsh r3, r2
        blr

instead of:

_baz:
        srwi r2, r4, 24
        rlwimi r2, r3, 8, 0, 23
        srwi r2, r2, 9
        extsh r3, r2
        blr

This implements CodeGen/PowerPC/sign_ext_inreg1.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36212 91177308-0d34-0410-b5e6-96231b3b80d8