Don't treat SIMD::Float preferentially

Don't treat SIMD::Float preferentially

Intermediate::Scalar was defined as RValue<SIMD::Float>, even though it
can hold integers as well. Use abstract Reactor values instead which
don't have a statically defined type.

This change does not yet avoid bitcasting on access. The EmitLoad()
implementation still assumes values come in as float.

Bug b/128539387

Change-Id: I18f449ebf68db7ddd81679d6d028911e6c02fc38
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26868
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>