[mlir][SCF] Do not access erased operation in `scf.while` lowering (#150741)

Do not access the erased `scf.while` operation in the lowering pattern.
That won't work anymore in a One-Shot Dialect Conversion and triggers a
use-after-free sanitizer error.

After the One-Shot Dialect Conversion refactoring, a
`ConversionPatternRewriter` will behave more like a normal
`PatternRewriter`.

GitOrigin-RevId: cccde9b2b1aa05f2797b29cba209c7642b7ac0f6