[llvm] Add an InstCount observation space.

[ChrisCummins]

This adds new observation spaces that expose the -instcount pass
values. The -instcount pass counts the number of instructions of each
type in a program, along with the total number of instructions, total
number of blocks, and total number of functions.

There are four new observation spaces: InstCount, which returns the
feature vector as a numpy array, InstCountDict, which returns the
values as a dictionary of named features, and InstCountNorm and
InstCountNormDict, which are the same as above but the counts are
instead normalized to the total number of instructions in the program.

Example usage:

>>> import gym
>>> import compiler_gym
>>> env = gym.make("llvm-v0")
>>> env.observation_space = "InstCountDict"
>>> env.reset("cBench-v0/crc32")
{'TotalInstsCount': 196, 'TotalBlocksCount': 29,
'TotalFuncsCount': 13, 'RetCount': 5, 'BrCount': 24,
'SwitchCount': 0, 'IndirectBrCount': 0, 'InvokeCount': 0,
'ResumeCount': 0, 'UnreachableCount': 0, 'CleanupRetCount': 0,
'CatchRetCount': 0, 'CatchSwitchCount': 0, 'CallBrCount': 0,
'FNegCount': 0, 'AddCount': 5, 'FAddCount': 0, 'SubCount': 0,
'FSubCount': 0, 'MulCount': 0, 'FMulCount': 0, 'UDivCount': 0,
'SDivCount': 0, 'FDivCount': 0, 'URemCount': 0, 'SRemCount': 0,
'FRemCount': 0, 'ShlCount': 0, 'LShrCount': 3, 'AShrCount': 0,
'AndCount': 3, 'OrCount': 1, 'XorCount': 8, 'AllocaCount': 24,
'LoadCount': 51, 'StoreCount': 38, 'GetElementPtrCount': 5,
'FenceCount': 0, 'AtomicCmpXchgCount': 0, 'AtomicRMWCount': 0,
'TruncCount': 1, 'ZExtCount': 5, 'SExtCount': 0, 'FPToUICount': 0,
'FPToSICount': 0, 'UIToFPCount': 0, 'SIToFPCount': 0,
'FPTruncCount': 0, 'FPExtCount': 0, 'PtrToIntCount': 0,
'IntToPtrCount': 0, 'BitCastCount': 0, 'AddrSpaceCastCount': 0,
'CleanupPadCount': 0, 'CatchPadCount': 0, 'ICmpCount': 10,
'FCmpCount': 0, 'PHICount': 0, 'CallCount': 13, 'SelectCount': 0,
'UserOp1Count': 0, 'UserOp2Count': 0, 'VAArgCount': 0,
'ExtractElementCount': 0, 'InsertElementCount': 0,
'ShuffleVectorCount': 0, 'ExtractValueCount': 0,
'InsertValueCount': 0, 'LandingPadCount': 0, 'FreezeCount': 0}

The InstCount observation spaces are quick to compute and
lightweight. They have similar computational complexity as Autophase.

Fixes #149.

[ChrisCummins]

Target //tests/llvm/service:ObservationSpacesTest was failing with a load-time error on macOS due to missing symbol __ZN4llvm24DisableABIBreakingChecksE. I've decided to remove the test since it overlaps entirely with the tests run in //tests/llvm:observation_spaces_test.