[interp] Kill more instructions (mono/mono#17226)

[interp] Kill more instructions (mono/mono#17226)

* [interp] Replace STLOC to dead local with MINT_POP

* [interp] Optimize MINT_POP

It was only used for 0 and 1 values. The implementation for values larger than 1 was broken anyway.

* [interp] Remove unnecessary usage of MINT_POP1

When storing a static field from a CEE_STFLD instruction.

* [interp] Prevent optimizing away the stack, after a branch

We know the contents of the stack following a branch, so we could have optimized away some instructions preceding the branch. We shouldn't do this because the code we are branching to expects a certain stack size.

* [interp] Clear instructions which push unused values on stack

* [interp] Properly initialize stack when pushing local

Commit migrated from https://github.com/mono/mono/commit/165c114aaa68a52c5aa10ae1df93816f71061840