RISC-V: Improve style to work around PR 60994 in host compiler.

RISC-V: Improve style to work around PR 60994 in host compiler.

This simple patch allows me to build a cross-compiler to riscv using
older versions of RedHat's system compiler.  The issue is PR c++/60994
where g++ doesn't like the same name (demand_flags) to be used by both
a variable and a (enumeration) type, which is also undesirable from a
(GNU) coding style perspective.  One solution is to rename the type
to demand_flags_t, but a less invasive change is to simply use another
identifier for the problematic local variable, renaming demand_flags
to dflags.

2023-12-02  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/riscv/riscv-vsetvl.cc (csetvl_info::parse_insn): Rename
	local variable from demand_flags to dflags, to avoid conflicting
	with (enumeration) type of the same name.