HLSL: emit OpLoopMerge Unroll for HLSL [unroll]

[danginsburg]

I believe that currently [unroll] in the HLSL does not result in OpLoopMerge Unroll in the SPIR-V. Please add support for this, it is a feature relied upon by a lot of HLSL for performance.

[gwihlidal]

This is definitely a critical optimization for us, as well

[johnkslang]

This is listed in #362, where I have bumped it to the top of the list (reflecting priority order).

The following is outside the scope of glslang, but I trust you are also lobbying IHVs to respect the SPIR-V hints for this?

[ghost]

I wonder if IHVs may often unroll a loop even without an explicit hint.

[ghost]

John, definitely want those hints to work in GLSL source as well.