[Local] Do not ignore zexts in salvageDebugInfo, PR45923

Summary:
When salvaging a dead zext instruction, append a convert operation to
the DIExpressions of the debug uses of the instruction, to prevent the
salvaged value from being sign-extended.

I confirmed that lldb prints out the correct unsigned result for "f" in
the example from PR45923 with this changed applied.

rdar://63246143

Reviewers: aprantl, jmorse, chrisjackson, davide

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80034