Follow-up to https://github.com/llvm/llvm-project/pull/143113.

Following up on a comment on
https://github.com/llvm/llvm-project/pull/142355.
Updated other instances in the file as well.

@jayfoad

Code ported from SelectionDAG::ComputeNumSignBits

Code ported from SelectionDAG::computeKnownBits.

- add KnownFPClass analysis to GISelValueTrackingPass
- add MI pattern for `m_GIsFPClass`

It was using the full size of the vector as the SrcBitWidth. This patch
changes the code to split G_ASSERT_ZEXT away from the others (G_INTTOPTR
/ G_PTRTOINT / G_ZEXT / G_TRUNC) which are simpler, and make the code
match the SDAG equivalent.

This forwards the KnownBits / SignBits from the scalar operand through
to the vector elements. The value is implicitly truncated if it is
larger than the vector element size. The change in
GISelValueTracking::computeNumSignBits allows scalable sign-bits using a
single element demanded elts, and brings it in line with the code in
ValueTracking and SDAG (and GISelValueTracking::getKnownBits). That was
the main motivation for adding this opcode, to prevent scalable vector
asserts.

The code is similar to computeKnownBits and the code in
SelectionDAG::ComputeNumSignBits.

The code is taken from SelectionDAG::computeKnownBits.