[ValueTracking] Fix a misuse of APInt in GetPointerBaseWithConstantOffset

GetPointerBaseWithConstantOffset include this code, where ByteOffset
and GEPOffset are both of type llvm::APInt :

  ByteOffset += GEPOffset.getSExtValue();

The problem with this line is that getSExtValue() returns an int64_t, but
the += matches an overload for uint64_t. The problem is that the resulting
APInt is no longer considered to be signed. That in turn causes assertion
failures later on if the relevant pointer type is > 64 bits in width and
the GEPOffset was negative.

Changing it to

  ByteOffset += GEPOffset.sextOrTrunc(ByteOffset.getBitWidth());

resolves the issue and explicitly performs the sign-extending
or truncation. Additionally, instead of asserting later if the result
is > 64 bits, it breaks out of the loop in that case.

See also
 https://reviews.llvm.org/D24729
 https://reviews.llvm.org/D24772

This commit must be merged after D38662 in order for the test to pass.

Patch by Michael Ferguson <mpfergu@gmail.com>.

Reviewers: reames, sanjoy, hfinkel

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D38501

llvm-svn: 350395