[TargetLowering] Teach BuildUDIV to take advantage of leading zeros in the dividend.

[TargetLowering] Teach BuildUDIV to take advantage of leading zeros in the dividend.

If the dividend has leading zeros, we can use them to reduce the
size of the multiplier and avoid the fixup cases.

This patch is for scalars only, but we might be able to do this
for vectors in a follow up.

Differential Revision: https://reviews.llvm.org/D140750