[GISel] Add new GISel combiners for G_SELECT

https://reviews.llvm.org/D83833

Patch adds two new GICombinerRules for G_SELECT. The rules include:
combining selects with undef comparisons into their first selectee value,
and to combine away selects with constant comparisons. Patch additionally
adds a new combiner test for the AArch64 target to test these new G_SELECT
combiner rules and the existing select_same_val combiner rule.

Patch by  mkitzan