[mlir][spirv] Do not truncate i/f64 -> i/f32 in SPIRVConversion

This truncation can be unexpected and break program behavior.
Dedicated emulation passes should be used instead.

Also rename pass options to "emulate-lt-32-bit-scalar-types".

Fixes: https://github.com/llvm/llvm-project/issues/57917

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D137115