Allow writing assembler instruction format with a layer turning it into DR IR

I thought we already had a long-standing feature request for this but I can't find it. The idea is to allow the tool writer, when dynamically generating sequences of instructions, to write them in conventional assembler format and have a layer that converts them into DR IR. While the INSTR_CREATE_ macros try to look like assembly, they are still bulky and have a lot of boilerplate.

The issue names no files or tests; start by examining the INSTR_CREATE_ macros and the DR IR instruction-generation entry points. Define the conventional assembler format and the conversion layer's supported scope, then verify that dynamically generated instruction sequences produce equivalent DR IR with less boilerplate.

Written by the indexing model from the issue text.