Clear converters map after X86 Domain Reassignment to avoid crashes

Summary:
As reported in PR37264, in some cases the X86 Domain Reassignment
`runOnMachineFunction()` is called twice.  Because it only deletes the
`.second` members of its `InstrConverterBaseMap`, and does not clean up
the map itself, this can lead to double frees and crashes.

Use `DeleteContainerSeconds()` instead, so the `Converters` map can
safely be reinitialized and its members re-deleted for each X86 Domain
Reassignment pass.

Reviewers: guyblank, craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46425

llvm-svn: 332176