The size of a reg_float isn't in the descr file, so ego doesn't know.
PowerPC and SPARC are the only arches with floating-point registers in
their descr files. PowerPC and SPARC registers can hold both 4-byte
and 8-byte floats, so I want ego to do both sizes.
This might break our SPARC code expander because ego doesn't know that
8-byte values take 2 registers in SPARC. (So ego might allocate too
many registers and deallocate too much stack space.) We don't build
the SPARC code expander, and its descr file is already wrong: its list
of register save costs is too short, so ego will read past the end of
the array.
This commit doesn't fix the problem with ego and PowerPC ncg. Right
now, ncg refuses to put 4-byte floats in registers, but ego expects
them to get registers and deallocates their stack space. So ncg emits
programs that use the deallocated space, and the values of 4-byte
floats become corrupt.
