[Assembler] Report multiple near misses for invalid instructions

[Assembler] Report multiple near misses for invalid instructions

The current table-generated assembly instruction matcher returns a
64-bit error code when matching fails. Since multiple instruction
encodings with the same mnemonic can fail for different reasons, it uses
some heuristics to decide which message is important.

This heuristic does not work well for targets that have many encodings
with the same mnemonic but different operands, or which have different
versions of instructions controlled by subtarget features, as it is hard
to know which encoding the user was intending to use.

Instead of trying to improve the heuristic in the table-generated
matcher, this patch changes it to report a list of near-miss encodings.
This list contains an entry for each encoding with the correct mnemonic,
but with exactly one thing preventing it from being valid. This thing
could be a single invalid operand, a missing target feature or a failed
target-specific validation function.

The target-specific assembly parser can then report an error message
giving multiple options for instruction variants that the user may have
been trying to use. For example, I am working on a patch to use this for
ARM, which can give this error for an invalid instruction for ARMv6-M:

  <stdin>:8:3: error: invalid instruction, multiple near-miss encodings found
    adds r0, r1, #0x8
    ^
  <stdin>:8:3: note: for one encoding: instruction requires: thumb2
    adds r0, r1, #0x8
    ^
  <stdin>:8:16: note: for one encoding: expected an integer in range [0, 7]
    adds r0, r1, #0x8
                 ^
  <stdin>:8:16: note: for one encoding: expected a register in range [r0, r7]
    adds r0, r1, #0x8
                 ^

This also allows the target-specific assembly parser to apply its own
heuristics to suppress some errors. For example, the error "instruction
requires: arm-mode" is never going to be useful when targeting an
M-profile architecture (which does not have ARM mode).

This patch just adds the target-independent mechanism for doing this,
all targets still use the old mechanism. I've added a bit in the
AsmParser tablegen class to allow targets to switch to this new
mechanism. To use this, the target-specific assembly parser will have to
be modified for the change in signature of MatchInstructionImpl, and to
report errors based on the list of near-misses.

Differential revision: https://reviews.llvm.org/D27620

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314774 91177308-0d34-0410-b5e6-96231b3b80d8