[update_cc_test_checks] Support 'clang | opt | FileCheck'

Some clang lit tests use a pipeline of the form

// RUN: %clang [args] -O0 %s | opt [specific optimizations] | FileCheck %s

to make the expected test output depend on as few optimization phases
as possible, for stability. But when you write a RUN line of this
form, you lose the ability to use update_cc_test_checks.py to
automatically generate the expected output, because it only supports
two-stage pipelines consisting of '%clang | FileCheck' (or %clang_cc1).

This change extends the set of supported RUN lines so that pipelines
with an invocation of `opt` in the middle can still be automatically
handled.

To implement it, I've adjusted `get_function_body()` so that it can
cope with an arbitrary sequence of intermediate pipeline commands. But
the code that decides which RUN lines to consider is more
conservative: it only adds clang | opt | FileCheck to the set of
supported lines, because I didn't want to accidentally include some
other kind of line that doesn't output IR at all.

(Also in this commit is the minimal change to make this script work at
all, after r373912 added an extra parameter to `add_ir_checks`.)

Reviewers: MaskRay, xbolva00

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68406

llvm-svn: 374287