[InlineCost][TargetTransformInfo][AMDGPU] Consider cost of alloca instructions in the caller (1/2)

On AMDGPU, alloca instructions have penalty that can
be avoided when SROA is applied after inlining.

This patch introduces the default implementation of
TargetTransformInfo::getCallerAllocaCost.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D149740