Make all unnamed RegisterClass TreePatternNodes typed MVT::i32.

A register class can appear as a leaf TreePatternNode with and without a
name:

  (COPY_TO_REGCLASS GPR:$src, F8RC)

In a named leaf node like GPR:$src, the register class provides type
information for the named variable represented by the node. The TypeSet
for such a node is the set of value types that the register class can
represent.

In an unnamed leaf node like F8RC above, the register class represents
itself as a kind of immediate. Such a node has the type MVT::i32,
we'll never create a virtual register representing it.

This change makes it possible to remove the special handling of
COPY_TO_REGCLASS in CodeGenDAGPatterns.cpp.