[ValueTracking] Simplify the known bits code for constant vectors a little.

[ValueTracking] Simplify the known bits code for constant vectors a little.

Neither of these cases really require a temporary APInt outside the loop. For the ConstantDataSequential case the APInt will never be larger than 64-bits so its fine to just call getElementAsAPInt. For ConstantVector we can get the APInt by reference and only make a copy where the inversion is needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316265 91177308-0d34-0410-b5e6-96231b3b80d8