Leave the primary InstBuilderBase trait alone, but add an alternative
InstInserterBase trait that can be implemented instead by builders that
always allocate new instructions with dfg.make_inst().
Any implementation of InstInserterBase can be used as an instruction
builder by wrapping it in an InsertBuilder. The InsertBuilder type adds
additional functionality via the with_results() method which makes it
possible to override the result values on the instruction that is built.
The motivation for this shuffle is that the with_result() functionality
can now be reused by different kinds of instruction builders, as long as
they insert new instructions. So ReplaceBuilder doesn't get
with_results().
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