instcombine: alloca: Canonicalize scalar allocation array size

As a follow-up to r232200, add an `-instcombine` to canonicalize scalar
allocations to `i32 1`.  Since r232200, `iX 1` (for X != 32) are only
created by RAUWs, so this shouldn't fire too often.  Nevertheless, it's
a cheap check and a nice cleanup.

llvm-svn: 232202