Work around glslang issue 988

[tangent-vector]

The basic bug there is that if you have a member of struct type in a uniform block and then pass a reference to that member directly to a call:

struct Foo { vec4 bar; };
uniform U { Foo foo; };

void main() { doSomething(foo); }

then glslang generates invalid SPIR-V which seems to cause an issue for some drivers.

This change works around the problem by detecting cases where an argument to a function call is a reference to uniform block member (of struct type) and then rewrites the code to move that value to a temporary before the call.