[llvm][TypeSize] Fix addition/subtraction in TypeSize. (#72979)

It seems TypeSize is currently broken in the sense that:

  TypeSize::Fixed(4) + TypeSize::Scalable(4) => TypeSize::Fixed(8)

without failing its assert that explicitly tests for this case:

  assert(LHS.Scalable == RHS.Scalable && ...);

The reason this fails is that `Scalable` is a static method of class
TypeSize,
and LHS and RHS are both objects of class TypeSize. So this is
evaluating
if the pointer to the function Scalable == the pointer to the function
Scalable,
which is always true because LHS and RHS have the same class.

This patch fixes the issue by renaming `TypeSize::Scalable` ->
`TypeSize::getScalable`, as well as `TypeSize::Fixed` to
`TypeSize::getFixed`,
so that it no longer clashes with the variable in
FixedOrScalableQuantity.

The new methods now also better match the coding standard, which
specifies that:
* Variable names should be nouns (as they represent state)
* Function names should be verb phrases (as they represent actions)