Fix SPIR-V for HLSL EvaluateAttribute* of interpolants in structs

Fix SPIR-V for HLSL EvaluateAttribute* of interpolants in structs

Generate load of interpolant for first operand to GLSLstd450
InterpolateAt* SPIR-V ops. This allows the interpolants to
propagate from the input struct in the wrapper around main
into the shader during HLSL legalization. A new pass has been
added to legalization which will remove the load and replace
with the pointer of the load to create valid external
interpolate op.

Fixes #2584