IBM Z: Fix "+fvm" constraint with long doubles
When a long double is passed to an asm statement with a "+fvm"
constraint, a LRA loop occurs. This happens, because LRA chooses the
widest register class in this case (VEC_REGS), but the code generated
by s390_md_asm_adjust() always wants FP_REGS. Mismatching register
classes cause infinite reloading.
Fix by treating "fv" constraints as "v" in s390_md_asm_adjust().
gcc/ChangeLog:
* config/s390/s390.c (f_constraint_p): Treat "fv" constraints
as "v".
gcc/testsuite/ChangeLog:
* gcc.target/s390/vector/long-double-asm-fprvrmem.c: New test.