[SystemZ] Improve AsmParser register parsing

The idea is to make sure that:

(1) "register expected" is restricted to cases where ParseRegister()
    is called and the token obviously isn't a register.

(2) "invalid register" is restricted to cases where a register-like "%..."
    sequence is found, but the "..." makes no sense.

(3) the generic "invalid operand for instruction" is used in cases where
    the wrong register type is used (GPR instead of FPR, etc.).

(4) the new "invalid register pair" is used if the register has the right type,
    but is not a valid register pair.

Testing of (1)-(3) is now restricted to regs-bad.s.  It uses a representative
instruction for each register class to make sure that only registers from
that class are accepted.

(4) is tested by both regs-bad.s (which checks all invalid register pairs)
and insn-bad.s (which tests one invalid pair for each instruction that
requires a pair).

While there, I changed "Number" to "Num" for consistency with the
operand class.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182643 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Improve AsmParser handling of invalid instructions

Previously, an invalid instruction like:

	foo     %r1, %r0

would generate the rather odd error message:

....: error: unknown token in expression
	foo     %r1, %r0
		^

We now get the more informative:

....: error: invalid instruction
	foo     %r1, %r0
	^

The same would happen if an address were used where a register was expected.
We now get "invalid operand for instruction" instead.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182644 91177308-0d34-0410-b5e6-96231b3b80d8