These will not only be useful for hipe_regalloc_prepass, but also, after
the introduction of a mk_move/2 (or similar) callback, for the purpose
of range splitting.

Since the substitution needed to case over all the instructions, a new
module, hipe_x86_subst, was introduced to the x86 backend.

Due to differences in the 'jtab' field of a #jmp_switch{} between x86
and amd64, it regrettably needed to be duplicated to hipe_amd64_subst.

There is little point offering LSRA for x86 if we're still going to call
hipe_graph_coloring_regalloc for the floats. In particular, all
allocators except LSRA allocates an N^2 interference matrix, making them
unusable for really large functions.

and correct the name of another, erroneously spelt, option in the process.

OTP-10106
OTP-10107