[InstCombineCasts] Fix checks in sext->lshr->trunc pattern.

[InstCombineCasts] Fix checks in sext->lshr->trunc pattern.

The comment says to avoid the case where zero bits are shifted into the truncated value,
but the code checks that the shift is smaller than the truncated value instead of the
number of bits added by the sign extension. Fixing this allows a shift by more than the
value size to be introduced, which is undefined behavior, so the shift is capped at the
value size minus one, which has the expected behavior of filling the value with the sign
bit.

Patch by Jacob Young!

Differential Revision: https://reviews.llvm.org/D32285

llvm-svn: 302548