Try to avoid duplication among generated assert constants in `.llir` file

[anmyachev]

Discovered in #2790

Example from test_scan_layouts[True-1-src_layout10-64-32]:

@assertFunc_447 = internal addrspace(1) constant [8 x i8] c"unknown\00"
@assertFile_447 = internal addrspace(1) constant [87 x i8] c".../pytest-0/test_scan_layouts_True_1_src_l0/test_scan_layouts.ttgir\00"
@assertMessage_447 = internal addrspace(1) constant [18 x i8] c"overflow detected\00"

According to the example above, there are more than 400 constants with the same values.

[victor-eds]

Going through review. Will address comments and merge this week.