optimize vector fneg of bitcasted integer value

optimize vector fneg of bitcasted integer value

This patch allows a vector fneg of a bitcasted integer value to be optimized in the same way that we already optimize a scalar fneg. If the integer variable is a constant, we can precompute the result and not require any logic ops.

This patch is very similar to a fabs patch committed at r214892.

Differential Revision: http://reviews.llvm.org/D4852

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215646 91177308-0d34-0410-b5e6-96231b3b80d8