Improve type safety around float constants

Instead of passing floating point constants to the vector support library as
compiler-side floats, pass them as APFloats instead.  This reduces the duration
during which these constants are semantically represented as floats on the host
side and are subject to fast-math-like behavior.  This is especially important
in cases where the exact bit representation of the floating point constant is
significant, but also makes progress towards ensuring that e.g. build XLA with
-ffast-math does not change the IR we generate.

PiperOrigin-RevId: 185611301