[flang] Allow more argument types in some specific intrinsic

Fix flang-compiler/f18#724.
Allow all type kinds for arguments in restricted specific conversion
intrinisc (no warning, this is ubiquitous).
Allow MAX/MIN restricted intrinsic (AMAX0...) to be replaced by the
related generic foolowed by a type conversion to the expected result
type of the specific. Emit a warning because xlf and ifort are doing so
but pgfortran is converting the arguments instead.

Original-commit: flang-compiler/f18@c07adb94ed2b2b46284d6a4a4b23c4189ea1d936
Reviewed-on: https://github.com/flang-compiler/f18/pull/749
Tree-same-pre-rewrite: false