Add script to compile to intermediate IR for easier pass test authoring (#5101)

This script automates some operations to make it easier to write IR tests:
  1. Gets the pass list for an HLSL compilation using -Odump
  2. Compiles HLSL with -fcgl and outputs to intermediate IR
  3. Collects list of passes before the desired pass and adds
     -hlsl-passes-pause to write correct metadata
  4. Invokes dxopt to run passes on -fcgl output and write bitcode result
  5. Disassembles bitcode to .ll file for use as a test
  6. Inserts RUN line with -hlsl-passes-resume and desired pass