[X86][BMI1]: X86DAGToDAGISel: select BEXTR from x & ((1 << nbits) + (-1)) pattern

Summary:
Trivial continuation of D52304.
While this pattern is not canonical, we do select it in the BZHI case,
so this should not be any different.

Reviewers: RKSimon, craig.topper, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52348

llvm-svn: 344902