[MachineSink] Use SkipPHIsAndLabels for sink insertion points

For AMDGPU the insertion point for a block may not be the first
non-PHI instruction.  This happens when a block contains EXEC
mask manipulation related to control flow (converging lanes).

Use SkipPHIsAndLabels to determine the block insertion point
so that the target can skip any block prologue instructions.

Reviewed By: rampitec, ruiling

Differential Revision: https://reviews.llvm.org/D119399