InstCombine rule to fold truncs whose value is available

We can fold truncs whose operand feeds from a load, if the trunc value
is available through a prior load/store.

This change is from: http://reviews.llvm.org/D21246, which folded the
trunc but missed the bitcast or ptrtoint/inttoptr required in the RAUW
call, when the load type didnt match the prior load/store type.

Differential Revision: http://reviews.llvm.org/D21791

llvm-svn: 274853