[EarlyCSE] detect equivalence of selects with inverse conditions and commuted operands (PR41101)

This is 1 of the problems discussed in the post-commit thread for:
rL355741 / http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190311/635516.html
and filed as:
https://bugs.llvm.org/show_bug.cgi?id=41101

Instcombine tries to canonicalize some of these cases (and there's room for improvement
there independently of this patch), but it can't always do that because of extra uses.
So we need to recognize these commuted operand patterns here in EarlyCSE. This is similar
to how we detect commuted compares and commuted min/max/abs.

Differential Revision: https://reviews.llvm.org/D60723

llvm-svn: 358523