inference: fix #42148, run find_throw_blocks once before inference

[aviatesk]

With this commit, we mark src.ssaflags before inference, and optimizer
will just observe them.

[vtjnash]

This is triggering an error in the Serialization test

$ make Serialization
    JULIA test/Serialization                                                                                   
Test      (Worker) | Time (s) | GC (s) | GC % | Alloc (MB) | RSS (MB) 
Serialization  (1) |        started at 2021-09-08T17:27:38.640
Internal error: encountered unexpected error in runtime:
BoundsError(a=Array{UInt8, (0,)}[], i=(1,))                                                                    
jl_bounds_error_ints at /data/vtjnash/julia/src/rtutils.c:194                                    
getindex at ./array.jl:904 [inlined]                                                                           
get_curr_ssaflag at ./compiler/inferencestate.jl:382 [inlined] 
abstract_call_gf_by_type at ./compiler/abstractinterpretation.jl:34
...