[AsmPrinter] Treat a narrowing PtrToInt like Trunc

When printing assembly for PtrToInt, AsmPrinter::lowerConstant
incorrectly assumed that if PtrToInt was not converting to an
int with exactly the same number of bits, it must be widening
to a larger int. But this isn't necessarily true; PtrToInt can
also shrink the size, which is useful when you want to produce
a known 32-bit pointer on a 64-bit platform (on x86_64 ELF
this yields a R_X86_64_32 relocation).

The old behavior of falling through to the widening case for a
narrowing PtrToInt yields bogus assembly code like this, which
fails to assemble because the no-op bit and it accidentally
creates is not a valid relocation:

```
        .long   a&-1
```

The fix is to treat a narrowing PtrToInt exactly the same as
it already treats Trunc: just emit the expression and let
the assembler deal with truncating it in the appropriate way.

Patch by Mat Hostetter <mjh@fb.com>.

Differential Revision: https://reviews.llvm.org/D61325

llvm-svn: 361508