[X86] X86DAGToDAGISel::matchBitExtract(): pattern a: truncation awareness

Summary:
Finally tying up loose ends here.

The problem is quite simple:
If we have pattern `(x >> start) &  (1 << nbits) - 1`,
and then truncate the result, that truncation will be propagated upwards,
into the `and`. And that isn't currently handled.

I'm only fixing pattern `a` here,
the same fix will be needed for patterns `b`/`c` too.

I *think* this isn't missing any extra legality checks,
since we only look past truncations. Similary, i don't think
we can get any other truncation there other than i64->i32.

Reviewers: craig.topper, RKSimon, spatel

Reviewed By: craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62786

llvm-svn: 364417