InstSimplify: Optimize away pointless comparisons

(X & INT_MIN) ? X & INT_MAX : X  into  X & INT_MAX
(X & INT_MIN) ? X : X & INT_MAX  into  X
(X & INT_MIN) ? X | INT_MIN : X  into  X
(X & INT_MIN) ? X : X | INT_MIN  into  X | INT_MIN

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224669 91177308-0d34-0410-b5e6-96231b3b80d8
Split executeShTest into two parts so that it can be better leveraged by libc++

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224672 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify the code

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224673 91177308-0d34-0410-b5e6-96231b3b80d8