[SLP] Consider alternatives for cost of select instructions.

Some architectures do not have general vector select instructions (e.g.
AArch64). But some cmp/select patterns can be vectorized using other
instructions/intrinsics.

One example is using min/max instructions for certain patterns.

This patch updates the cost calculations for selects in the SLP
vectorizer to consider using min/max intrinsics.

This patch does not change SLP vectorizer's codegen itself to actually
generate those intrinsics, but relies on the backends to lower the
vector cmps & selects. This keeps things simple on the SLP side and
works well in practice for AArch64.

This exposes additional SLP vectorization opportunities in some
benchmarks on AArch64 (-O3 -flto).

Metric: SLP.NumVectorInstructions

Program                                        base    slp     diff
 test-suite...ications/JM/ldecod/ldecod.test   502.00  697.00  38.8%
 test-suite...ications/JM/lencod/lencod.test   1023.00 1414.00 38.2%
 test-suite...-typeset/consumer-typeset.test    56.00   65.00  16.1%
 test-suite...6/464.h264ref/464.h264ref.test   804.00  822.00   2.2%
 test-suite...006/453.povray/453.povray.test   3335.00 3357.00  0.7%
 test-suite...CFP2000/177.mesa/177.mesa.test   2110.00 2121.00  0.5%
 test-suite...:: External/Povray/povray.test   2378.00 2382.00  0.2%

Reviewed By: RKSimon, samparker

Differential Revision: https://reviews.llvm.org/D89969