[LowerConstantIntrinsics] avoid crashing on alloca with unexpected operand type

The test here is reduced from the fuzzer-generated crasher in:
https://llvm.org/PR50023
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33395

I don't know if this is the best or complete solution, but the
zext of the i42 type appears to match the behavior if we run a
weird type example like this through the IR optimizer with -O1.

Differential Revision: https://reviews.llvm.org/D100766