[GlobalISel] Change widenScalar of G_FCONSTANT to mutate into G_CONSTANT.

Widening a G_FCONSTANT by extending and then generating G_FPTRUNC doesn't produce
the same result all the time. Instead, we can just transform it to a G_CONSTANT
of the same bit pattern and truncate using a plain G_TRUNC instead.

Fixes https://github.com/llvm/llvm-project/issues/56454

Differential Revision: https://reviews.llvm.org/D129743