This patch adds direct code-gen support for a faster MOD intrinsic for
REAL types. Flang has maintained and keeps maintaining a high-precision
implementation of the MOD intrinsic as part of the Fortran runtime. With
the -ffast-real-mod flag, users can opt to avoid calling into the
Fortran runtime, but instead trigger code-gen that produces faster code
by avoiding the runtime call, at the expense of potentially risking bit
cancelation by having the compiler use the MOD formula a specified by
ISO Fortran.
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