[AMDGPU] Make BVH isel consistent with other MIMG opcodes

Suffix opcodes with _gfx10.
Remove direct references to architecture specific opcodes.
Add a BVH flag and apply this to diassembly.
Fix a number of disassembly errors on gfx90a target caused by
previous incorrect BVH detection code.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D108117