spirv-opt: "unroll" loops with a count of 1

It would be nice for spirv-opt to systematically "unroll" loops with a constant iteration count of 1 (without having to specify `[[unroll]]` which is not available in glsl by default).

e.g.:

```
#version 310 es
precision mediump float;
precision mediump int;
layout(location = 0) uniform float count;
layout(location = 0) out vec4 fragColor;
void main() {
vec4 r = vec4(0.0);
for (float i=0.0 ; i<1.0 ; i+=1.0) {
r += count;
}
for (int j=0 ; j<1 ; j+=1) {
r += count;
}
fragColor = vec4(r);
}
```

When doing:

```
./glslangValidator --target-env opengl test.frag -o test.spv
./spirv-opt -O test.spv -o test.opt.spv
./spirv-cross --es test.opt.spv
```

Yields:

```
#version 310 es
precision mediump float;
precision highp int;

layout(location = 0) uniform float count;

layout(location = 0) out vec4 fragColor;

void main()
{
vec4 _60;
_60 = vec4(0.0);
for (float _57 = 0.0; _57 < 1.0; )
{
_60 += vec4(count);
_57 += 1.0;
continue;
}
vec4 _59;
_59 = _60;
for (mediump int _58 = 0; _58 < 1; )
{
_59 += vec4(count);
_58++;
continue;
}
fragColor = _59;
}
```

Note: these loops above go away entirely if the iteration count is 0, so the optimizer handles that, it would seem natural to handle the special case of one iteration too.