[Scalarizer] Treat values from unreachable blocks as undef

Summary:
When scalarizing PHI nodes we might try to examine/rewrite
InsertElement nodes in predecessors. If those predecessors
are unreachable from entry, then the IR in those blocks could
have unexpected properties resulting in infinite loops in
Scatterer::operator[].
By simply treating values originating from instructions in
unreachable blocks as undef we do not need to analyse them
further.

This fixes PR41723.

Reviewers: bjope

Reviewed By: bjope

Subscribers: bjope, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70171