Move alignTo computation inside the if.

This is an improvement when compiling with llvm. llvm doesn't inline
the call to insert, so the align is always executed and shows up in
the profile.

With gcc the call to insert is inlined and the align computation moved
and done only if needed.

With this patch we explicitly only compute it if it is needed.

In the two tests with debug info, the speedup was

scylla
  master 3.008959365
  patch  2.932080942 1.02621974786x faster

firefox
  master 6.709823604
  patch  6.592387227 1.01781393795x faster

In all others the difference was in the noise.

llvm-svn: 284249