[GlobalISel] Don't skip adding predicate matcher

This patch fixes a bug which skipped
adding predicate matcher for a pattern in many cases.
For example, if predicate is Load and
its memoryVT is non-null then the loop
continues and never reaches to the end which
adds the predicate matcher. This patch moves the
matcher addition to the top of the loop
so that it gets added regardless of contextual checks
later in the loop.
Other way to fix this issue is to remove all "continue" statements
in checks and let the loop continue till end.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D83034