Weaken constraint on VMADC overlap; apply new constraint to all comparisons

* Treat vrgather scalar register index as unsigned

The index in scalar register should also be unsigned, so express it
explicitly.

Signed-off-by: Weiwei Wang <weiwei.wangx@outlook.com>

* Replace imm with uimm for shift instructions

When an immediate is used as a shift, it is treated as unsigned, so
use uimm explictly to reflect it.

Signed-off-by: Weiwei Wang <weiwei.wangx@outlook.com>

A literal reading of the current text would imply it should be signed (as that is the default and vrgather's description says nothing to the contrary), but negative immediates are useless on this instruction, as they would (after sign extension) be larger than 2^31 and thus exceed VLMAX on any implementation and just splat a constant 0 into the destination.

FWIW, binutils currently treats it as unsigned.

FIX(inst-table.adoc) asciidoctor-pdf Prawn::Errors::CannotFit.

Constrain vms*f to require vstart = 0

Rationale is similar to that for vmpopc/vmfirst, and avoids need for
a constraint on `vd`.

Closes #201.

Replaced widening multiply-subtract instructions with widening signed/unsigned multiply-add instructions.

Made integer single-width multiply-subtract opcode names and function match the scalar/vector floating-point operation names/functions.

Canonicalized ordering of multiply-add operands in various instruction descriptions.  No change in functionality.

Add note about vsetvl with rd=x0; explain why it is useful

Resolves #199

Closes #194.

Closes #193.

Closes #192.

Closes #169,

Closes #198

Changed behavior of mask-logical instructions to write boolean values (zero-extended 0 or 1 value) instead of performing bitwise logical operations.
Also changed name of pseudo-instruction vmmv.m to vmcpy.m to signify that it no longer does a bitwise copy.
Closes #190.

Fixed vmerge synatx to treat mask input  as regular vector, and then to treat unmasked variants as  real instructions instead of pseudo-instructions.
Closes #189.

Closes #165.