[TTI, AArch64] Allow the cost model analysis to test vector reduce intrinsics
This patch considers the experimental vector reduce intrinsics in the default
implementation of getIntrinsicInstrCost. The cost of these intrinsics is
computed with getArithmeticReductionCost and getMinMaxReductionCost. This patch
also adds a test case for AArch64 that indicates the costs we currently compute
for vector reduce intrinsics. These costs are inaccurate and will be updated in
a follow-on patch.
Differential Revision: https://reviews.llvm.org/D44489