[TypePromotion] Fix sext vs zext in promoted constant

As pointed out in #55342, given non-canonical IR with multiple
constants, we check the second operand in isSafeWrap, but can promote
both with sext. Fix that as suggested by @craig.topper by ensuring we
only extend the second constant if multiple are present.

Fixes #55342

Differential Revision: https://reviews.llvm.org/D125294