[ValueTracking] Support vscale in computeConstantRange()

Add support for vscale in computeConstantRange(), based on
vscale_range attributes. This allows simplifying based on the
precise range, rather than a KnownBits approximation (which will
be off by a factor of two for the usual case of a power of two
upper bound).

Differential Revision: https://reviews.llvm.org/D146217