Avoid hard-coding some labels in the bitfield generation

[sampsyo]

Some labels for fields in instructions are hard-coded:

riscv.fyi/src/lib/loadInstructions.ts

Lines 146 to 148 in d24668c

	if (hi === 31 && lo === 25) label = `funct7=${bits}`;
	else if (hi === 14 && lo === 12) label = `funct3=${bits}`;
	else if (hi === 6 && lo === 0) label = `opcode=${bits}`;

Let's avoid that and somehow try to derive all bitfield labels from the underlying data.