Dear all,

as reported, the Fortran standard requires all actual argument
expressions to be evaluated (e.g. F2018:15.5.3).

There were two cases for intrinsic MERGE where we failed to do so:

- non-constant mask; Steve provided the patch

- constant scalar mask; we need to be careful to simplify only if
  the argument on the "other" path is known to be constant so that
  it does not have side-effects and can be immediately removed.

The latter change needed a correction of a sub-test of testcase
merge_init_expr_2.f90, which should not have been simplified
the way the original author assumed.  I decided to modify the
test in such way that simplification is valid and provides
the expect pattern.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald