[WinEH] Fix eh.exceptionpointer intrinsic lowering

Summary:
Some shared code for handling eh.exceptionpointer and eh.exceptioncode
needs to not share the part that truncates to 32 bits, which is intended
just for exception codes.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13747

llvm-svn: 250588