Fix bug 30945- [AVX512] Failure to flip vector comparison to remove not mask instruction

Fix bug 30945- [AVX512] Failure to flip vector comparison to remove not mask instruction

adding new optimization opportunity by adding new X86ISelLowering pattern. The test case was shown in https://llvm.org/bugs/show_bug.cgi?id=30945.

Test explanation:
Select gets three arguments mask, op and op2. In this case, the Mask is a result of ICMP. The ICMP instruction compares (with equal operand) the zero initializer vector and the result of the first ICMP.

In general, The result of "cmp eq, op1, zero initializers" is "not(op1)" where op1 is a mask. By rearranging of the two arguments inside the Select instruction, we can get the same result. Without the necessary of the middle phase ("cmp eq, op1, zero initializers").

Missed optimization opportunity:
vpcmpled %zmm0, %zmm1, %k0
knotw %k0, %k1

can be combine to
vpcmpgtd %zmm0, %zmm2, %k1

Reviewers:
1. delena
2. igorb

Commited after check all
Differential Revision: https://reviews.llvm.org/D27160

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289653 91177308-0d34-0410-b5e6-96231b3b80d8