[SDAG] Allow scalable vectors in ComputeNumSignBits

This is a continuation of the series of patches adding lane wise support for scalable vectors in various knownbit-esq routines.

The basic idea here is that we track a single lane for scalable vectors which corresponds to an unknown number of lanes at runtime. This is enough for us to perform lane wise reasoning on many arithmetic operations.

Differential Revision: https://reviews.llvm.org/D137141