[llvm] [EarlyIfConversion] Fix the logic to determine predictable branch. (PR #92405)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

@fhahn It looks like AArch64 sometimes only puts condition code into `Cond`, while the branch instruction is reading the `$nzcv`. For an example see `AArch64/ifcvt-select.ll` test. How is the code supposed to figure out if the branch is predictable only based on `Cond` (as returned by `analyzeBranch`)?

Would it be better to use `analyzeBranchPredicate` which should give us the pointer to the `MachineInstr` defining the condition? Then, we can check if all operands of the condition-defining instruction are loop-invariant?

Currently `analyzeBranchPredicate` for `AArch64` leaves `ConditionDef = nullptr`, so this part would need to be implemented.

@fhahn If these changes sound reasonable,  I can work on it.

https://github.com/llvm/llvm-project/pull/92405