xtensa: Change the splitting of D[IF]mode constant assignments to be implemented...

xtensa: Change the splitting of D[IF]mode constant assignments to be implemented in xt_largeconst instead of define_split

This patch moves the process of splitting D[IF]mode constant assignments
into SImode ones from the define_split implementation after reloading to
processing within the "xt_largeconst" target-specific pass.  It also
converts SFmode constant assignments into bit-equivalent SImode ones.

This allows these assignments to be processed by the "constantsynth"
optimization, which will be reimplemented later.

gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h
	(xtensa_split_DI_reg_imm): Remove.
	* config/xtensa/xtensa.cc (xtensa_split_DI_reg_imm): Remove.
	(split_DI_SF_DF_const): New worker function.
	(do_largeconst): Add a call to split_DI_SF_DF_const() to the insn
	enumeration loop.
	* config/xtensa/xtensa.md (movdi): Remove split code when the
	source is constant.
	(movdi_internal): Add a new constraint pair (a, Y) to the second
	of the existing constraint alternatives.
	(The auxiliary define_split for movdi_internal): Remove.