[mlir][TOSA] Do not access erased operation (#148374)
This commit fixes two occurrences where an erased op was accessed at a
later point of time. 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`.
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Co-authored-by: Markus Böck <markus.boeck02@gmail.com>
