Handle the options passed to llvm opt and llvm llc

[chentong319]

Issue #2950 discovered the issue in passing options llvm. There are several problems:

1. The string of the options has to be split into vector of strings for the command format.
2. Conflict in options. So far, the optimization level is the only example. The level specified with particular step should have higher priority than that for the whole compiler. Thus, I can avoid multiple "-O#" error.
3. modify the comments for mllvm, xopt, and xllc to make it easy for user to pass multiple values to these options

I tested with some examples.

[tungld]

Options of -mllvm is for both llvm opt and llc, but their options are not the same. New option, -mllvmopt and -mllvmllc, are introduced to pass specific option to opt or llc respectively.

Look like -mllvmopt and -mllvmllc are similar to two existing options -Xopt and -Xllc.

[chentong319]

Options of -mllvm is for both llvm opt and llc, but their options are not the same. New option, -mllvmopt and -mllvmllc, are introduced to pass specific option to opt or llc respectively.

Look like -mllvmopt and -mllvmllc are similar to two existing options -Xopt and -Xllc.

That's true. The only difference is that each value of opt and xllc has the same limitation of mllvm: only one option is allowed. I removed the new llvmopt and llvmllc, and modified the option explanation for opt and xllc.

[AlexandreEichenberger]

LGTM, thanks for the very clear text in the options, with examples. That should be a model for future options!

[tungld]

LGTM.

[jenkins-droid]

Jenkins Linux amd64 Build #15721 [push] Handle the options passe... started at 12:13

[jenkins-droid]

Jenkins Linux s390x Build #15724 [push] Handle the options passe... started at 13:13

[jenkins-droid]

Jenkins Linux ppc64le Build #14751 [push] Handle the options passe... started at 13:24

[jenkins-droid]

Jenkins Linux amd64 Build #15721 [push] Handle the options passe... passed after 1 hr 13 min

[jenkins-droid]

Jenkins Linux s390x Build #15724 [push] Handle the options passe... passed after 1 hr 41 min

[jenkins-droid]

Jenkins Linux ppc64le Build #14751 [push] Handle the options passe... passed after 2 hr 22 min