Function holds all the blocks used by the function.
Moved various function-specific methods from SpirvShader to this new Function inner class.

This change is currently a pure-refactor (no change in behavior), but is required for function inlining in SpirvShader.

Bug: b/133213304
Change-Id: I50c7ecca8ce518d8df054c2461410c0acd4e1f52
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33352
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>