[AMDGPU][NFC] Refactor emitEntryFunctionPrologue

Remove dead code and factor repeated conditions out into a single check.
Rename and move code to make it more obvious what is running only for
entry functions. Simplify function arguments to make it clearer what the
relevant inputs are. Make flat scratch init accept an MBB iterator and
move it to where it was logically being emitted within the prologue.

These changes will make a future update to the calling convention
simpler.

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75092