This change lays the foundation for implementing branches and loops.  In
SPIR-V, every block starts with an OpLabel (which identifies the block
and serves as the branch target to that block), and ends in a branch.
An `if` for example implies the existence of four blocks, the header
including code leading up to the if, the true and false blocks and the
"merge" block, including the code following the if-else blocks.

This change includes support code for generating function code split in
blocks, even though only one block is currently used.

Bug: angleproject:4889
Change-Id: I10f96b78b6f00c20bc7f9c81e854ab5543bf8fde
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2929659Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>