Compiler: fixed is_a? transform may cause an exception

[rockwyc992]

Fixed #4364

Something.some_method.is_a?(Const)

This code may trigger an exception about enclosing call.
This will be transformed to below:

Const === Something.some_method

The compiler checks enclosing call of Something.some_method twice, and causes an exception.
So, cloning the ASTNode could solve this bug.

[rockwyc992]

What?
Is there anyone who could restart this build?

Travis CI error log

==> Installing llvm 
==> Downloading https://homebrew.bintray.com/bottles/llvm-4.0.0_1.sierra.bottle.
Error: Failed to download resource "llvm"

[refi64]

Just close and re-open the PR. It'll automatically restart the build.

[rockwyc992]

@kirbyfan64 Thanks!
But someone has restarted this build, and it build passed.

[mverzilli]

Thanks! Looks good to me, I'm just running a couple of quick benchmarks and unless I find something odd there I'll merge it soon.