i386: Tweak ix86_mode_can_transfer_bits to restore bootstrap on RHEL.

This minor patch, very similar to one posted and approved previously at
https://gcc.gnu.org/pipermail/gcc-patches/2024-July/657229.html is
required to restore builds on systems using gcc 4.8 as a host compiler.
Using the enumeration constants E_SFmode and E_DFmode avoids issues with
SFmode and DFmode being "non-literal types in constant expressions".

2024-08-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.cc (ix86_mode_can_transfer_bits): Use E_?Fmode
	enumeration constants in switch statement.