Use TcMType.growThetaTyVars (which works) rather than TcSimplify.growPreds (which doesn't)

I think this got left behind when we simplified and improved TcSimplify.  The effect
was that we had a function like
   class P a b | a -> b
   class Q b c | b -> c

   f :: (P a b, Q b c) => a -> a

and were were failing to quanitfy over 'c', even though it is (indirectly) determined
by 'a'.

This make Programatica fail to compile: Trac #7147

MERGED from commit de07bf26