[MemoryBuiltins] Allow truncation in visitAllocaInst()

Summary:
Solves PR33689.

If the pointer size is less than the size of the type used for the array
size in an alloca (the <ty> type below) then we could trigger the assert in
the PR. In that example we have pointer size i16 and <ty> is i32.

<result> = alloca [inalloca] <type> [, <ty> <NumElements>] [, align <alignment>]

Handle the situation by allowing truncation as well as zero extension in
ObjectSizeOffsetVisitor::visitAllocaInst().

Also, we now detect overflow in visitAllocaInst(), similar to how it was
already done in visitCallSite().

Reviewers: craig.topper, rnk, george.burgess.iv

Reviewed By: george.burgess.iv

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D35003

llvm-svn: 307754