Preserve load alignment in instcombine transformations. I've been unable to

create a testcase where this matters.  The select+load transformation only
occurs when isSafeToLoadUnconditionally is true, and in those situations,
instcombine also changes the underlying objects to be aligned.  This seems
like a good idea regardless, and I've verified that it doesn't pessimize
the subsequent realignment.

llvm-svn: 94850