Remove `>` and `>=` comparisons from IR

#### Feature
The condition codes for performing the `>` and `>=` comparisons on integers should be removed from the Cranelift IR

#### Benefit
Makes the IR more strongly normalizing. At the moment, there are two equivalent ways of specifying the same computation: `x > y` and `y < x` are semantically equivalent but have different representations in the IR.

Having only one possible representation would mean fewer cases to consider when writing lowering/optimization code, and fewer tests have to be performed at runtime when legalizing/optimizing programs.

#### Implementation
Continue to accept the `sgt`, `sge`, `ugt` and `uge` condition codes when consuming textual IR, but convert them to their swapped forms and swap the arguments to `icmp` during parsing. Update legalization/optimization code to fix any codegen regressions.

#### Alternatives
Do nothing