Drop `scf.if` if it only has an `scf.yield`.

reprod.py

from xdsl.dialects import scf, arith, func
from xdsl.ir import Block, Region
from xdsl.context import Context
from xdsl.pattern_rewriter import PatternRewriteWalker, PatternRewriter, GreedyRewritePatternApplier
from xdsl.transforms.canonicalization_patterns.scf import IfPropagateConstantCondition
from xdsl.printer import Printer

ctx = Context()
ctx.load_dialect(scf.Scf)
ctx.load_dialect(arith.Arith)
ctx.load_dialect(func.Func)

true_op = arith.ConstantOp.from_int_and_width(1, 1)
if_op = scf.IfOp(true_op.result, [], [Block()])
if_op.true_region.block.add_op(scf.YieldOp())
func_op = func.FuncOp("test", ([], []), Region([Block([true_op, if_op])]))

print("----- before canonicalization:")
printer = Printer()
printer.print_op(func_op)
print("\n")

pattern = IfPropagateConstantCondition()
applier = GreedyRewritePatternApplier([pattern])
walker = PatternRewriteWalker(applier)
walker.rewrite_module(func_op)

print("----- after canonicalization:")
printer.print_op(func_op)
print("\n")

"""
----- before canonicalization:
func.func @test() {
  %0 = arith.constant true
  scf.if %0 {
  }
}

----- after canonicalization:
func.func @test() {
  %0 = arith.constant true
}
"""