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.