[llvm-reduce] Introduce operands-to-args pass.

Instead of setting operands to undef as the "operands" pass does,
convert the operands to a function argument. This avoids having to
introduce undef values into the IR which have some unpredictability
during optimizations.

For instance,

    define void @func() {
    entry:
      %val = add i32 32, 21
      store i32 %val, i32* null
      ret void
    }

is reduced to

    define void @func(i32 %val) {
    entry:
      %val1 = add i32 32, 21
      store i32 %val, i32* null
      ret void
    }

(note that the instruction %val is renamed to %val1 when printing
the IR to avoid ambiguity; ideally %val1 would be removed by dce or the
instruction reduction pass)

Any call to @func is replaced with a call to the function with the
new signature and filled with undef. This is not ideal for IPA passes,
but those out-of-scope for now.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D111503