Simplify MULTI_SUBSCRIPT implementation

The MULTI_SUBSCRIPT code in evaluate_subexp_standard has a comment
saying that perhaps the EVAL_SKIP handling is incorrect.  This patch
simplifies this code.  In particular, it precomputes all the indices
in a separate loop and removes some complicated flow-control.

Tested using the gdb.modula2 and gdb.dlang test suites, as these are
the only parsers that emit MULTI_SUBSCRIPT.

gdb/ChangeLog
2020-12-27  Tom Tromey  <tom@tromey.com>

	* eval.c (evaluate_subexp_standard) <case MULTI_SUBSCRIPT>:
	Simplify.