[Split GEP] handle trunc() in separate-const-offset-from-gep pass.

Let separate-const-offset-from-gep pass handle trunc() when it calculates
constant offset relative to base. The pass itself may insert trunc()
instructions when it canonicalises array indices to pointer-size integers
and needs to handle trunc() in order to evaluate the offset.

Differential Revision: https://reviews.llvm.org/D46732

llvm-svn: 332142