wazevo: add bit count instruction Popcnt

[evacchi]

Moves further #1496 and follows-up to #1635.

We introduce support to popcnt with the required dependencies for its implementation: we use vector instructions CNT and UADDLV as we do in the current compiler. This PR also implements movToVec, movFromVec and it lays down the first steps to extend further with the missing related operations in vecMisc, vecLanes (originally unimplemented).

[mathetake]

9e944dc is landed, so could you rebase onto main before making this ready btw

[evacchi]

moved forward, a bit of cleanup, still needs tests and docs :) pushing for feedback 🙏

[mathetake]

yeah this is right on the track! 💯

[mathetake]

OK, VecRRR is not the right place for UADDLV, but instead you should use vecLanes:

	// vecLanes represents a vector instruction across lanes.
	vecLanes

which should be encoded following "Advanced SIMD across lanes" in https://developer.arm.com/documentation/ddi0596/2020-12/Index-by-Encoding/Data-Processing----Scalar-Floating-Point-and-Advanced-SIMD?lang=en

[mathetake]

Awesome!