[AMDGPU] Better support for VMEM soft clauses in GCNHazardRecognizer

VMEM soft clauses only contain VMEM and FLAT instructions. Teaching
GCNHazardRecognizer::checkSoftClauseHazards that other kinds of
instructions will naturally break the clause means there are far fewer
cases where it has to insert an s_nop instruction to forcibly break the
clause.

Differential Revision: https://reviews.llvm.org/D79353