[AMDGPU][CodeGen] Improve handling of memcpy for -Os/-Oz compilations (#87632)

We had some instances when LLVM would not inline fixed-count memcpy and
ended up
attempting to lower it a a libcall, which would not work on AMDGPU as
the
address space doesn't meet the requirement, causing compiler crash.

The patch relaxes the threshold used for -Os/-Oz compilation so we're
always allowed
to inline memory copy functions.

This patch basically does the same thing as
https://reviews.llvm.org/D158226 for
AMDGPU.

Fix #88497.