MSL: Work around internal compiler error from mulhi.

[squidbus]

Proposed fix for #2385

This introduces a helper function to perform Op*MulExtended instructions. The helper takes the following template parameters:

• The type of the result container.
• The type of the operands and result container fields. This is needed because OpSMulExtended can be used with unsigned types but will perform signed multiplication. The inputs are cast to signed to perform correct multiplication, and the results need to be cast back to unsigned afterwards in order to satisfy Metal/C++ rules for initializer list types.
• The type the function will use to multiply arguments, based on the instruction.

Updated the tests for these changes, and the shader in the linked issue now compiles without error.

[CLAassistant]

All committers have signed the CLA.

[HansKristian-Work]

On a quick glance this is fine, it can also be considered a cleanup, which is nice. I'll look at it in more detail tomorrow.