[flang] Readability improvement in binary->decimal conversion

Tweak binary->decimal conversions to avoid an integer multiplication
in a hot loop to improve readability and get a minor (~5%) speed-up.
Use native integer division by constants for more readability, too,
since current build compilers seem to optimize it correctly now.
Delete the now needless temporary work-around facility in
Common/unsigned-const-division.h.

Differential revision: https://reviews.llvm.org/D88604