[X86] Add target combine rule to select ADDSUB instructions from a build_vector

This patch teaches the backend how to combine a build_vector that implements
an 'addsub' between packed float vectors into a sequence of vector add
and vector sub followed by a VSELECT.

The new VSELECT is expected to be lowered into a BLENDI.
At ISel stage, the sequence 'vector add + vector sub + BLENDI' is
pattern-matched against ISel patterns added at r211427 to select
'addsub' instructions.
Added three more ISel patterns for ADDSUB.

Added test sse3-avx-addsub-2.ll to verify that we correctly emit 'addsub'
instructions.