RFA: -mfpmath=sse -fpic vs double constants

[Paolo Bonzini]

> Like this; untested, but pretty obvious.  Ok for mainline?

David Edelsohn disapproved this on IRC because PPC benefits from this 
(also performance-wise).

It is possible to change compress_float_constant to do the change only 
if {cost_of_load_single}+{cost_of_extend} <= {cost_of_load_double}.

Unfortunately a change to compress_float_constant needs auditing on all 
targets that explicitly test the cost of FLOAT_EXTEND.  These are:
- alpha
- i386
- mips
- rs6000, whose cost can be lowered to 0 for DFmode
- sparc
- vax, whose cost have to be checked anyway according to a comment.

Paolo