[FunctionAttrs] Unconditionally perform argument attribute inference in the...

[FunctionAttrs] Unconditionally perform argument attribute inference in the first function-attrs pass

Summary:
  Argument attributes like NoAlias and ReadOnly could affect memoryssa and thus earlyCSE in the function simplification pipeline.
https://reviews.llvm.org/D145210 adjusted PostOrderFunctionAttrs placement and caused the argument attributes not referred for the use
in the pipeline. This work (initiated by @nikic) unconditionally performs argument attribute inference in the first function-attrs pass.

Reviewers:
  aeubanks and nikic

Differential Revision:
  https://reviews.llvm.org/D156397