Completely rewrite tblgen's type inference mechanism,

Completely rewrite tblgen's type inference mechanism,
changing the primary datastructure from being a
"std::vector<unsigned char>" to being a new TypeSet class
that actually has (gasp) invariants!

This changes more things than I remember, but one major
innovation here is that it enforces that named input
values agree in type with their output values.

This also eliminates code that transparently assumes (in
some cases) that SDNodeXForm input/output types are the
same, because this is wrong in many case.

This also eliminates a bug which caused a lot of ambiguous
patterns to go undetected, where a register class would
sometimes pick the first possible type, causing an
ambiguous pattern to get arbitrary results.

With all the recent target changes, this causes no
functionality change!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98534 91177308-0d34-0410-b5e6-96231b3b80d8