[AMDGPU] Reintroduce CC exception for non-inlined functions in Promote Alloca limits

This is basically a partial revert of https://reviews.llvm.org/D145586 ( fd1d6087 )

D145586 was originally introduced to help with SWDEV-363662, and it did, but
it also caused a 25% drop in performance in
some MIOpen benchmarks where, it seems,
functions are inlined more conservatively.

This patch restores the pre-D145586 behavior
for PromoteAlloca: functions with a non-entry CC
have a 32 VGPRs threshold, but only if the function
is not marked with "alwaysinline".

A good number of AMDGPU code makes uses of
the AMDGPUAlwaysInline pass anyway, so in our
backend "alwaysinline" seems very common.

This change does not affect SWDEV-363662 (the motivating issue for introducing D145586).

Fixes SWDEV-399519

Reviewed By: rampitec, #amdgpu

Differential Revision: https://reviews.llvm.org/D150551