[LV] Avoid redundant operations manipulating masks

[LV] Avoid redundant operations manipulating masks

The Loop Vectorizer generates redundant operations when manipulating masks:
AND with true, OR with false, compare equal to true. Instead of relying on
a subsequent pass to clean them up, this patch avoids generating them.

Use null (no-mask) to represent all-one full masks, instead of a constant
all-one vector, following the convention of masked gathers and scatters.

Preparing for a follow-up VPlan patch in which these mask manipulating
operations are modeled using recipes.

Differential Revision: https://reviews.llvm.org/D35725

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309558 91177308-0d34-0410-b5e6-96231b3b80d8