Situations may arise leading to negative `NumElements` argument of an
`alloca` instruction. In this case the `NumElements` is treated as a
large unsigned value. Such large arrays may cause the size constant to
overflow during code generation under 32 bit mode, leading to a crash.
This PR limits the constant's bit width to the width of the pointer on
the target. With this fix,
```
alloca i32, i32 -1
```
and
```
alloca [4294967295 x i32], i32 1
```
generates the exact same PowerPC assembly code under 32 bit mode.
