[APInt] In udivrem, remember the bit width in a local variable so we don't reread...

[APInt] In udivrem, remember the bit width in a local variable so we don't reread it from the LHS which might be aliased with Quotient or Remainder.

This helped the compiler generate better code for the single word case. It was able to remember that the bit width was still a single word when it created the Remainder APInt and not create code for it possibly being multiword.

llvm-svn: 302952