[SCEV] Support rewriting ZExt expressions with loop guard info.

So far, applying loop guard information has been restricted to
SCEVUnknown. In a few cases, like PR40961 and PR52464, this leads to
SCEV failing to determine tight upper bounds for the backedge taken
count.

This patch adjusts SCEVLoopGuardRewriter and applyLoopGuards to support
re-writing ZExt expressions.

This is a first step towards fixing  PR40961 and PR52464.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D113577