In accordance with https://github.com/llvm/llvm-project/issues/123569

This also adds a default constructor and a few uses of it.

The new token type is used in #67006 for implementing convergence
control tokens in GMIR.

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)

The current isScalable function requires a user to call isVector before
hand in order to avoid an assertion failure in the case that the LLT is
not a vector.

This patch addds helper functions that allow a user to query whether the
LLT is fixed or scalable, not wanting an assertion failure in the case
that the LLT was never a vector in the first place.

Those fixes were taken from https://reviews.llvm.org/D137338

Before restoring `CodeGen/LowLevelType`, rename this to `LowLevelTypeUtils`.

Differential Revision: https://reviews.llvm.org/D148768

I initially attempted this in 5a95be22d248be654b992dfb25e3850dbb182a14. It
was reverted in 81cbe0ca83c2f912ff612ddb65629a108197b0d1 since it
crashed GCC 5.3. That has dropped out of the list of supported host
compilers, so try again.