docs: ALU: drop to keyword from polyfill names

Most of the LLVM IR instructions come in the form of just an opcode
followed by operands, but some introduce an extra `to` keyword, e.g.
`zext` is called as follows:

```llvm
%X = zext i32 257 to i64
```

As discussed in the compiler skeleton PR[1], this naming duality make
the instructions-to-polyfills mapping more complex without adding any
benefits. Remove the `to` keyword from polyfill names. Affected
instructions: trunc, zext, sext, ptrtoint, inttoptr, bitcast.

--

[1] #74 (comment)

Signed-off-by: Wojciech Zmuda <[email protected]>