[AMDGPU] override isHighLatencyDef

SIMachineScheduler uses isHighLatencyInstruction with the same
sematincs, but TargetInstrInfo has virtual isHighLatencyDef
method, so override it instead.

Added FLAT to the list of high latency opcodes and a check for
mayLoad since stores are not technically high latency in terms
of data dependency.

This change did not produce any visible impact on our tests.

Differential Revision: https://reviews.llvm.org/D73582