[CostModel][X86] Add explicit vector select costs

Prior to SSE41 (and sometimes on AVX1), vector select has to be performed as a ((X & C)|(Y & ~C)) bit select.

Exposes a couple of issues with the min/max reduction costs (which only go down to SSE42 for some reason).

The increase pre-SSE41 selection costs also prevent a couple of tests from firing any longer, so I've either tweaked the target or added AVX tests as well to the existing SSE2 tests.

llvm-svn: 351685