Changes include:
    Fix SSA rewrite for nested loops.
    Add support for two new extensions:
    - SPV_NV_shader_subgroup_partitioned
    - SPV_EXT_descriptor_indexing
    Legalize OpImageTexelPointer
    Copy propagate arrays

Also update spirv-opt recipes.

Contains the following improvements:

Fixes 1341: Atomics (and other instructions with side-effects) removed.
Add folding for redundant add/sub/mul/div/mix operations
Add constant folding rules for floating-point comparison
Add folding of redundant OpSelect insns
Add folding of OpCompositeExtract and OpConstantComposite

One legalization case updated for a renumbered ID

Update SPIRV-Tools.  Relevant functional changes:
 - Optimizer enhancements:
   - ADCE now removes OpSwitch
   - Block merging occurs in more cases
 - Optimizer fixes:
   - Constant propagation (CCP): support matrix constants
   - #1199: Optimizer: Fix CCP: don't propagate spec constants.
   - #1203: Optimizer: Fix common uniform elim bug introduced by refactoring.
   - #1210: Optimizer: Aggressive dead code elimination: Fix 'break' identification.
   - #1212: Optimizer: Aggressive dead code elimination: Was skipping too many instructions.
   - #1214: Optimizer: Aggressive dead code elimination: Fix infinite loop.
   - #1228: Optimizer: Fix CCP: Handling of varying Phi nodes; was resulting in infinite loop.
   - #1245: Optimizer: Dead branch elimination: Avoid a null pointer dereference.
   - #1250: Optimizer: Dead branch elimination: Avoid spuriously reporting a change.
   - #1262: Support building on VisualStudio 2013 again

Update SPIRV-Headers, with "unified1" directory.

Updated one Glslang legalization test base result due to better block merging.

Optimizations used in HLSL legalization now sweep away unused types.

This fixes spirv-tools issue 989 where if-break can be incorrectly
deleted from a loop.

This includes an enhancement to propagate through nested structs.

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.