SelectionDAG: Teach ComputeKnownBits about VSCALE

This reverts commit 9b92f70d.  The issue
with the re-applied change was an implicit truncation due to the
multiplication.  Although the operations were converted to `APInt`, the
values were implicitly converted to `long` due to the typing rules.

Fixes: #59594

Differential Revision: https://reviews.llvm.org/D140347