fix ssa conversion for catch blocks

[Pangoraw]

the slotused function was not enough to account for all slot used by the catch block and its successors. With this change, ssa conversion keeps a live list of all catch 'branch' instructions and fetches the reaching definitions for slots at the location of these :catch instructions.

See as an example the added test where the ir is:

function f_try_catch(x)
    y = 0.
    try
        y = sqrt(x)
    catch

    end
    y
end
# Before (catch branches have no args since slotused(block(ir,2)) is empty)
#
# 1: (%1, %2)
#   %3 = try #2
#   %4 = catch 2
#   %5 = Main.sqrt(%2)
#   %6 = catch 2
#   %7 = end try
#   br 3 (%5)
# 2: (%8)
#   %9 = end try
#   %10 = pop exception %3
#   br 3 (%8)
# 3: (%11)
#   return %11
#
# After (catch branches have args corresponding to the reaching definition of y)
#
# 1: (%1, %2)
#   %3 = try #2
#   %4 = catch 2 (0.0)
#   %5 = Main.sqrt(%2)
#   %6 = catch 2 (%5)
#   %7 = end try
#   br 3 (%5)
# 2: (%8)
#   %9 = end try
#   %10 = pop exception %3
#   br 3 (%8)
# 3: (%11)
#   return %11

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (55c315a) 73.29% compared to head (f728dc8) 73.61%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   73.29%   73.61%   +0.31%     
==========================================
  Files          16       16              
  Lines        1494     1512      +18     
==========================================
+ Hits         1095     1113      +18     
  Misses        399      399              

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[oxinabox]

can we add a test_throws UndefVarError ... broken=true for this? (and the one above)

[Pangoraw]

Good idea. It looks like @test_throws does not support broken=true yet so I had added @test try ... end broken=true instead.

[oxinabox]

this is purely just a clarification of the existing code, right? to rename v to slot
(A good clarification)

[Pangoraw]

Yes! In other parts of the package, v often refers to a Variable (SSA value) so I renamed to clarify.

[oxinabox]

This makes sense.
Add the test_broken and this should be good to merge