[SystemZ] Recognize RISBG opportunities involving a truncate

Summary:
Recognize RISBG opportunities where the end result is narrower than the
original input - where a truncate separates the shift/and operations.

The motivating case is some code in postgres which looks like:

	srlg	%r2, %r0, 11
	nilh	%r2, 255

Reviewers: uweigand

Author: RolandF

Differential Revision: http://reviews.llvm.org/D21452

llvm-svn: 273433