ConvertToCFG: convert "if" statements.

The condition of the "if" statement is an integer set, defined as a conjunction
of affine constraints.  An affine constraints consists of an affine expression
and a flag indicating whether the expression is strictly equal to zero or is
also allowed to be greater than zero.  Affine maps, accepted by `affine_apply`
are also formed from affine expressions.  Leverage this fact to implement the
checking of "if" conditions.  Each affine expression from the integer set is
converted into an affine map.  This map is applied to the arguments of the "if"
statement.  The result of the application is compared with zero given the
equality flag to obtain the final boolean value.  The conjunction of conditions
is tested sequentially with short-circuit branching to the "else" branch if any
of the condition evaluates to false.

Create an SESE region for the if statement (including its "then" and optional
"else" statement blocks) and append it to the end of the current region.  The
conditional region consists of a sequence of condition-checking blocks that
implement the short-circuit scheme, followed by a "then" SESE region and an
"else" SESE region, and the continuation block that post-dominates all blocks
of the "if" statement.  The flow of blocks that correspond to the "then" and
"else" clauses are constructed recursively, enabling easy nesting of "if"
statements and if-then-else-if chains.

Note that MLIR semantics does not require nor prohibit short-circuit
evaluation.  Since affine expressions do not have side effects, there is no
observable difference in the program behavior.  We may trade off extra
operations for operation-level parallelism opportunity by first performing all
`affine_apply` and comparison operations independently, and then performing a
tree pattern reduction of the resulting boolean values with the `muli i1`
operations (in absence of the dedicated bit operations).  The pros and cons are
not clear, and since MLIR does not include parallel semantics, we prefer to
minimize the number of sequentially executed operations.

PiperOrigin-RevId: 223970248