[codegen][LLVM][bugfix] Specify argument to FastMathFlags setAllowContract

[slyubomirsky]

PR #9223 added a fast math flags setting to the LLVM codegen. The setting checks if the LLVM version is >= 6.0 and calls the below lines

    if (fast_math_contract) {
      fmf.setAllowContract();
    }

Recent LLVM versions have a default argument of true to llvm::FastMathFlags::setAllowContract(). However, LLVM 6.0 does not have a default argument for that method, so this fails to compile because it expects an explicit argument. This PR makes the argument explicit to be compatible with LLVM 6.0 (the version my system happened to have).

[slyubomirsky]

Please review @masahi (reviewer on PR #9223)

[AndrewZhaoLuo]

Can you set the other flags to true explicitly. It appears I missed this in 6.x (I swear they had default args): https://github.com/llvm/llvm-project/blob/release/6.x/llvm/include/llvm/IR/Operator.h

Edit: Actually nvrmind, I cant read, this LGTM

[slyubomirsky]

I can fix others too. I only fixed the one that was breaking my own build locally ;) Wow, how weird, that is literally the only one with an arg