[AMDGPU] Recompute liveness in SIOptimizeExecMaskingPreRA

Instead of ad-hoc updating liveness, recompute it completely for the affected register.

This does not affect any existing test and fixes an edge case that
caused a "Non-empty but used interval" error in the register allocator
due to how the pass updated liveranges. It created a "isolated" live-through
segment.

Overall this change just seems to be a net positive with no side effect observed. There may be a compile time impact but it's expected to be minimal.

Fixes SWDEV-388279

Reviewed By: critson

Differential Revision: https://reviews.llvm.org/D150105