Use `OpConversionPattern` instead of `OpRewritePattern` for operation
conversion during dialect conversion. This makes explicit and in-place
type conversions unnecessary, since `OpConversionPattern` already
properly converts operand types and provides them to the rewrite rule
through an operation adaptor.
The main contributions of this commit are the two class templates
`TypeConvertingReinstantiationPattern` and
`GenericOneToOneOpConversionPattern`.
The former allows for the definition of a simple replacement rule that
re-instantiates an operation after the types of its operands have been
converted. This is especially useful for type-polymorphic operations
during dialect conversion.
The latter allows for the definition of patterns, where one operation
needs to be replaced with a different operation after conversion of
its operands.
The default implementations for the class templates provide
conversions rules for operations that have a generic builder method
that takes the desired return type(s), the operands and (optionally) a
set of attributes. How attributes are discarded during a conversion
(either by omitting the builder argument or by passing an empty set of
attributes) can be defined through specialization of
`ReinstantiationAttributeDismissalStrategy`.
Custom replacement rules that deviate from the scheme above should be
implemented by specializing
`TypeConvertingReinstantiationPattern::matchAndRewrite()` and
`GenericOneToOneOpConversionPattern::matchAndRewrite()`.
