[framelowering] Skip dbg values when getting next/previous instruction.

Summary:
In mergeSPUpdates, debug values need to be ignored when getting the
previous element, otherwise debug data could have an impact on codegen.

In eliminateCallFramePseudoInstr, debug values after the erased element
could have an impact on codegen and should be skipped.

Closes PR31319 (https://llvm.org/bugs/show_bug.cgi?id=31319)

Reviewers: aprantl, MatzeB, mkuper

Subscribers: gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D27688

llvm-svn: 290955