irgen: Use getelementptr constant expression for global variables

[mewmew]

Clang seems to be using getelementptr constant expression for global variables, and getelementptr instructions for local variables. Mimic this behaviour to facilitate testing.

Relevant test cases:

• [x] testdata/extra/irgen/global_array_arg.c
• [x] testdata/extra/irgen/global_array_ident_use.c
• [x] testdata/extra/irgen/global_array_param.c
• [x] testdata/extra/irgen/global_index_expr_use.c
• [x] testdata/extra/irgen/global_index_expr_def.c

[mewmew]

Fixed as of commit dbec8c9.