Hoist the definition of getTypeSizeInBits to be inlinable and in the

Hoist the definition of getTypeSizeInBits to be inlinable and in the
header.

This method is called in the hot path for *many* passes, SROA is what
caught my interest. A common pattern is that which branch of the switch
should be taken is known in the callsite and so it is a very good
candidate for inlining and simplification. Moving it into the header
allows the optimizer to fold a lot of boring, repeatitive code in
callers of this routine.

I'm seeing pretty significant speedups in parts of SROA and I suspect
other passes will see similar speedups if they end up working with type
sizes frequently. I've not seen any significant growth of the binaries
as a consequence, but let me know if you see anything suspicious here.

llvm-svn: 177632