[flang][codegen] Keep primitive type for extractvalue and insertvalue

llvm.insertvalue and llvm.extractvalue need LLVM primitive type
for the indexing operands. While upstreaming the TargetRewrite pass the change
was made from i32 to index without knowing this restriction. This patch reverts
back the types used for indexing in the two ops created in this pass.

the error you will receive when lowering to LLVM IR with the current code
is the following:

```
 'llvm.insertvalue' op operand #1 must be primitive LLVM type, but got 'index'
```

Reviewed By: jeanPerier, schweitz

Differential Revision: https://reviews.llvm.org/D119253