[SimpleLoopUnswitch] Unswitch AND/OR conditions of selects

If a select's condition is a AND/OR, we can unswitch invariant operands.
This patch uses existing logic from unswitching AND/OR's for branch
conditions.

This patch fixes the Cost computation for unswitching selects to have
the cost of the entire loop, since unswitching selects do not remove
branches. This is required for this patch because otherwise, there are
cases where unswitching selects of AND/OR is beating out unswitching of
branches.

This patch also prevents unswitching of logical AND/OR selects. This
should instead be done by unswitching of AND/OR branch conditions.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D151677