[AMDGPU] Do not add debug locations to the code inside prologue

There is no real source location for code inside prologue as it is
generated by compiler but source locations are being added to code
inside prologue as a side effect of https://reviews.llvm.org/D99269
because buildSpillLoadStore() is using source location of the real
instruction in the basic block if any.

Fixes: SWDEV-307590

Reviewed By: scott.linder, sebastian-ne

Differential Revision: https://reviews.llvm.org/D113100