[AMDGPU] Prevent spills before exec mask is restored

Inline spiller can decide to move a spill as early as possible in the basic block.
It will skip phis and label, but we also need to make sure it skips instructions
in the basic block prologue which restore exec mask.

Added isPositionLike callback in TargetInstrInfo to detect instructions which
shall be skipped in addition to common phis, labels etc.

Differential Revision: https://reviews.llvm.org/D27997

llvm-svn: 292554