FMV priority is the returned value of a polymorphic function. On RISC-V
and X86 targets a 32-bit value is enough. On AArch64 we currently need
64 bits and we will soon exceed that. APInt seems to be a suitable
replacement for uint64_t, presumably with minimal compile time overhead.
It allows bit manipulation, comparison and variable bit width.

Adds sve-sm4 to reference FEAT_SVE_SM4 without specifically enabling
SVE2.

This patch transforms:

  X && *X == Y

to:

  X == Y

where X is of std::optional<T>, and Y is of T or similar.

Adds sve-sha3 to reference FEAT_SVE_SHA3 without specifically enabling
SVE2. The SVE2 requirement for AES, SHA3 and Bitperm is replaced with
SVE for non-streaming function.

The `targetFeatureToExtension` function used by
reconstructFromParsedFeatures only found positive `+FEATURE` strings,
but not negative `-FEATURE` strings. Extend the function to handle both
to fix `reconstructFromParsedFeatures`.

To deduce whether the optimization is legal we need to compare the target
features between caller and callee versions. The criteria for bypassing
the resolver are the following:

 * If the callee's feature set is a subset of the caller's feature set,
   then the callee is a candidate for direct call.

 * Among such candidates the one of highest priority is the best match
   and it shall be picked, unless there is a version of the callee with
   higher priority than the best match which cannot be picked from a
   higher priority caller (directly or through the resolver).

 * For every higher priority callee version than the best match, there
   is a higher priority caller version whose feature set availability
   is implied by the callee's feature set.

Example:

Callers and Callees are ordered in decreasing priority.
The arrows indicate successful call redirections.

  Caller        Callee      Explanation
=========================================================================
mops+sve2 --+--> mops       all the callee versions are subsets of the
            |               caller but mops has the highest priority
            |
     mops --+    sve2       between mops and default callees, mops wins

      sve        sve        between sve and default callees, sve wins
                            but sve2 does not have a high priority caller

  default -----> default    sve (callee) implies sve (caller),
                            sve2(callee) implies sve (caller),
                            mops(callee) implies mops(caller)

The 20204-12 ISA update release adds a new feature: FEAT_SSVE_BitPerm,
which allows the sve-bitperm instructions to run in streaming mode.

It also removes the requirement of FEAT_SVE2 for FEAT_SVE_BitPerm. The
sve2-bitperm feature is now an alias for sve-bitperm and sve2.

A new feature flag sve-bitperm is added to reflect the change that the
instructions under FEAT_SVE_BitPerm are supported if:
 on non streaming mode with FEAT_SVE2 and FEAT_SVE_BitPerm or
 in streaming mode with FEAT_SME and FEAT_SSVE_BitPerm

Currently, the more features a version has, the higher its priority is.
We are changing ACLE https://github.com/ARM-software/acle/pull/370 as
follows:

"Among any two versions, the higher priority version is determined by
 identifying the highest priority feature that is specified in exactly
 one of the versions, and selecting that version."