nir/builder: Add support for easily building control-flow

nir/builder: Add support for easily building control-flow

Each of the pop functions (and push_else) take a control flow parameter as
their second argument.  If NULL, it assumes that the builder is in a block
that's a direct child of the control-flow node you want to pop off the
virtual stack.  This is what 90% of consumers will want.  The SPIR-V pass,
however, is a bit more "creative" about how it walks the CFG and it needs
to be able to pop multiple levels at a time, hence the argument.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>