[DAGCombiner] Enhance (zext(setcc))

Current `v:t = zext(setcc x,y,cc)` will be transformed to `select x, y, 1:t, 0:t, cc`. It misses some opportunities if x's type size is less than `t`'s size. This patch enhances the above transformation.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D86687