[LIR] Partially revert r252926(NFC), which introduced a very subtle change.

In short, before r252926 we were comparing an unsigned (StoreSize) against an a
APInt (Stride), which is fine and well.  After we were zero extending the Stride
and then converting to an unsigned, which is not the same thing.  Obviously,
Stides can also be negative.  This commit just restores the original behavior.

AFAICT, it's not possible to write a test case to expose the issue because
the code already has checks to make sure the StoreSize can't overflow an
unsigned (which prevents the Stride from overflowing an unsigned as well).

llvm-svn: 260706