sparc: Clean up some code in base.isa.

sparc: Clean up some code in base.isa.

This includes the filterDoubles function which adds code to combine 32
bit values into doubles or 64 bit values for floating point, and the
splitOutImm function which detects if the code that implements an
instruction has a register and immediate variant, and generates code for
each.

Change-Id: I5524b9acd6e610b51fd91fe70276c34c23be9f85
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35235
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>