InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN

It is not safe to negate the smallest signed integer, doing so yields
the same number back.

This fixes PR20186.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212164 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Add a vector variant test for PR20186

No functional change, just adding more test coverage that was meant to
go in with r212164.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212165 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] Hoist an if-else chain on ISD opcodes (really designed for
switches) into a switch, and sink them into a dispatch function that can
return the result rather than awkward variable setting with breaks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212166 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Optimize x/INT_MIN to x==INT_MIN

The result of x/INT_MIN is either 0 or 1, we can just use an icmp
instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212167 91177308-0d34-0410-b5e6-96231b3b80d8
Fix missing const

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212168 91177308-0d34-0410-b5e6-96231b3b80d8
Move operator[] to DomTreeNodeBase

The comment in base even refers to it, but it was only defined
in the subclass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212169 91177308-0d34-0410-b5e6-96231b3b80d8