[X86][fast-isel] Fix select lowering.
The condition in selects is supposed to be i1.
Make sure we are just reading the less significant bit
of the 8 bits width value to match this constraint.

<rdar://problem/15651765>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197712 91177308-0d34-0410-b5e6-96231b3b80d8