[X86] Lower VSELECT into SHRUNKBLEND when we shrink the bits used into the
condition to match a blend.
This prevents optimizations that work on VSELECT to perform invalid
transformations. Indeed, the optimized condition does not match the vector
boolean content that is expected and bad things may happen.

This patch yields the exact same code on the whole test-suite + specs (-O3 and
-O3 -march=core-avx2), it improves one test case (vector-blend.ll) and fixes a
bug reduced in vselect-avx.ll.

<rdar://problem/18819506>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221429 91177308-0d34-0410-b5e6-96231b3b80d8
Allow L symbols in no_dead_strip sections.

If a section cannot be dead stripped, it is safe to use L symbols, since
the linker will keep all of it in the end.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221431 91177308-0d34-0410-b5e6-96231b3b80d8