LoongArch: Add fscaleb.{s,d} instructions as ldexp{sf,df}3

LoongArch: Add fscaleb.{s,d} instructions as ldexp{sf,df}3

This allows optimizing __builtin_ldexp{,f} and __builtin_scalbn{,f} with
-fno-math-errno.

IMODE is added because we can't hard code SI for operand 2: fscaleb.d
instruction always take the high half of both source registers into
account.  See my_ldexp_long in the test case.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (UNSPEC_FSCALEB): New unspec.
	(type): Add fscaleb.
	(IMODE): New mode attr.
	(ldexp<mode>3): New instruction template.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/fscaleb.c: New test.