Fold (x << c1)>> c2 into a single shift if the bits shifted out aren't used.

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

into:
_baz:
        srwi r2, r3, 1
        extsh r3, r2
        blr

on PPC, instead of:
_baz:
        slwi r2, r3, 8
        srwi r2, r2, 9
        extsh r3, r2
        blr

GCC produces:
_baz:
        srwi r10,r4,24
        insrwi r10,r3,24,0
        srawi r9,r3,24
        srawi r3,r10,9
        extsh r3,r3
        blr

This implements CodeGen/PowerPC/shl_elim.ll

llvm-svn: 36221