[MLIR][Mem2Reg] Ensure dominance of default value in regions (#193708)
When we promote an allocation, and a default value for a load from an
uninitialized slot is required, this value used to get inserted in the
same block as the allocation. However, in some cases, the default value
needs to be available in the predecessor blocks so that they can pass it
to the block of the allocation as an argument. For example, this is the
case for loops containing an allocation where the promoted value will
become and IV.
Make sure the default value is always available to all blocks by
creating it in the entry block of the region.
For reference, this is what used to be the output for the test. Note the
use of `%1`.
```
"func.func"() <{function_type = (f64) -> (), sym_name = "poison_insertion_point"}> ({
^bb0(%arg0: f64):
"cf.br"(%1)[^bb1] : (f64) -> ()
^bb1(%0: f64): // 2 preds: ^bb0, ^bb2
%1 = "ub.poison"() <{value = #ub.poison}> : () -> f64
%2 = "test.get"() : () -> i1
"cf.cond_br"(%2)[^bb2, ^bb3] <{operandSegmentSizes = array<i32: 1, 0, 0>}> : (i1) -> ()
^bb2: // pred: ^bb1
%3 = "test.get"() : () -> i1
%4 = "scf.if"(%3) ({
"scf.yield"(%arg0) : (f64) -> ()
}, {
"scf.yield"(%0) : (f64) -> ()
}) : (i1) -> f64
"test.use"(%4) : (f64) -> ()
"cf.br"(%4)[^bb1] : (f64) -> ()
^bb3: // pred: ^bb1
"func.return"() : () -> ()
}) : () -> ()
```
llvm-project
36d19f50 - [MLIR][Mem2Reg] Ensure dominance of default value in regions (#193708)
