instcombine: alloca: Limit array size type promotion

Move type promotion of the size of the array allocation to the end of
`simplifyAllocaArraySize()`.  This avoids promoting the type of the
array size if it's a `ConstantInt`, since the next -instcombine
iteration will drop it to a scalar allocation anyway.  Similarly, this
avoids promoting the type if it's an `UndefValue`, in which case the
alloca gets RAUW'ed.

This is NFC when considered over the lifetime of -instcombine, since
it's just reducing the number of iterations needed to reach fixed point.

llvm-svn: 232201