[AMDGPU] Add agpr_count to metadata and AsmParser

gfx90a allows the number of ACC registers (AGPRs) to be set
independently to the VGPR registers. For both HSA and PAL metadata, we
now include an "agpr_count" key to report the number of AGPRs set for
supported devices (gfx90a, gfx908, as determined by hasMAIInsts()).
This is collected from SIProgramInfo.NumAccVGPR for both HSA and PAL.
The AsmParser also now recognizes ".kernel.agpr_count" for supported
devices.

Differential Revision: https://reviews.llvm.org/D116140