Perform scalar constant propagation for FPTruncOp only if the resulting value can be represented without precision loss or rounding.
Example:
%cst = constant 1.000000e+00 : f32
%0 = fptrunc %cst : f32 to bf16
-->
%cst = constant 1.000000e+00 : bf16
Reviewed By: mehdi_amini
Differential Revision: https://reviews.llvm.org/D107518