[LV] Sink scalar operands of predicated instructions

[LV] Sink scalar operands of predicated instructions

When we predicate an instruction (div, rem, store) we place the instruction in
its own basic block within the vectorized loop. If a predicated instruction has
scalar operands, it's possible to recursively sink these scalar expressions
into the predicated block so that they might avoid execution. This patch sinks
as much scalar computation as possible into predicated blocks. We previously
were able to sink such operands only if they were extractelement instructions.

Differential Revision: https://reviews.llvm.org/D25632

llvm-svn: 285097