miri: don't mutate ecx in read_discriminant_value

[bjorn3]

This causes a borrowck error in oli-obk/priroda#8 when trying to read the discriminant of a value while the stack is borrowed immutably.

https://github.com/oli-obk/priroda/pull/8/files#diff-1336350dd7c33bda9f60eab418d60f2bR132

cc @oli-obk