This commit moves the information on whether a register is constant into
the Tablegen files to allow generating the implementaiton of
isConstantPhysReg(). I've marked isConstantPhysReg() as final in this
generated file to ensure that changes are made to tablegen instead of
overriding this function, but if that turns out to be too restrictive,
we can remove the qualifier.
This should be pretty much NFC, but I did notice that e.g. the AMDGPU
generated file also includes the LO16/HI16 registers now.
The new isConstant flag will also be used by D131958 to ensure that
constant registers are marked as call-preserved.