getType(), getObject() and getFunction() are only internally used, make them private.

Minor refactoring related to spirv function inlining.

Bug: b/133213304
Change-Id: I75f9f5488a8485161d534b56eb6530c2946bf49e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33353Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>