[TableGen] TypeInfer - Cache the legal types as TypeSetByHwMode

We were just caching the MVT set of legal types, then every call creating a new TypeSetByHwMode with it and passing it back on the stack. There's no need to do this - we can create and cache the whole TypeSetByHwMode once and return a const reference to it each time.

Additionally, TypeInfer::expandOverloads wasn't making use of the fact that the cache just contains a default mode containing all the types.

Saves up to 30secs in debug builds of x86 -gen-dag-isel.

Differential Revision: https://reviews.llvm.org/D50903

llvm-svn: 340042