[InstCombine] fold trunc ([lshr] (bitcast vector) ) --> extractelement (PR25543)

This is a fix for PR25543:
https://llvm.org/bugs/show_bug.cgi?id=25543

The idea is to take the existing fold of:
bitcast ( trunc ( lshr ( bitcast X))) --> extractelement (bitcast X)
( http://reviews.llvm.org/rL112232 )

And break it into less specific transforms so we'll catch more cases such as
the example in the bug report:
bitcast ( trunc ( lshr ( bitcast X))) -->
bitcast ( extractelement (bitcast X)) -->
extractelement (bitcast X)

Enabling patches for this change:
http://reviews.llvm.org/rL255399 (combine bitcasts)
http://reviews.llvm.org/rL255433 (canonicalize extractelement(bitcast X))

Differential Revision: http://reviews.llvm.org/D15392

llvm-svn: 255504