[ARM] Skip debug info in recomputeVPTBlockMask

The ARMLowOverheadLoops pass recalculates VPT block masks when it
converts VCMP's inside VPT blocks into VPT's. The function to do so
doesn't seem to handle debug info though, leading to invalid block
creation or asserts at compile time. Make sure the function skips any
debug info between the MVE instructions it inspects.

Differential Revision: https://reviews.llvm.org/D110564