TableGen: Allow !cast of records, cleanup conversion machinery

TableGen: Allow !cast of records, cleanup conversion machinery

Summary:
Distinguish two relationships between types: is-a and convertible-to.
For example, a bit is not an int or vice versa, but they can be
converted into each other (with range checks that you can think of
as "dynamic": unlike other type checks, those range checks do not
happen during parsing, but only once the final values have been
established).

Actually converting initializers between types is subtle: even
when values of type A can be converted to type B (e.g. int into
string), it may not be possible to do so with a concrete initializer
(e.g., a VarInit that refers to a variable of type int cannot
be immediately converted to a string).

For this reason, distinguish between getCastTo and convertInitializerTo:
the latter implements the actual conversion when appropriate, while
the former will first try to do the actual conversion and fall back
to introducing a !cast operation so that the conversion will be
delayed until variable references have been resolved.

To make the approach of adding !cast operations to work, !cast needs
to fallback to convertInitializerTo when the special string <-> record
logic does not apply.

This enables casting records to a subclass, although that new
functionality is only truly useful together with !isa, which will be
added in a later change.

The test is removed because it uses !srl on a bit sequence,
which cannot really be supported consistently, but luckily
isn't used anywhere either.

Change-Id: I98168bf52649176654ed2ec61a29bdb29970cfe7

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43753

llvm-svn: 326785