Add folding for redundant add/sub/mul/div/mix operations

This change implements instruction folding for arithmetic operations
that are redundant, specifically:

  x + 0 = 0 + x = x
  x - 0 = x
  0 - x = -x
  x * 0 = 0 * x = 0
  x * 1 = 1 * x = x
  0 / x = 0
  x / 1 = x
  mix(a, b, 0) = a
  mix(a, b, 1) = b

Cache ExtInst import id in feature manager

This allows us to avoid string lookups during optimization; for now we
just cache GLSL std450 import id but I can imagine caching more sets as
they become utilized by the optimizer.

Add tests for add/sub/mul/div/mix folding

The tests cover scalar float/double cases, and some vector cases.

Since most of the code for floating point folding is shared, the tests
for vector folding are not as exhaustive as scalar.

To test sub->negate folding I had to implement a custom fixture.