Fix RValue<T> construction from incorrect types.

Fix RValue<T> construction from incorrect types.

When constructing an RValue<T> from a Value, the types should match.
Else a bitcast is required.

Bug swiftshader:48

Change-Id: I5073091524d2f56681dab052c9f84a06b3be7b4f
Reviewed-on: https://swiftshader-review.googlesource.com/10908
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>