codegen quality: unnecessary function prologue and epilogue

[andrewrk]

zig version: 0.14.0-dev.2916+d916954be

export fn f(a: usize) usize {
    return a + 1;
}

zig build-obj -fno-llvm -fno-lld test.zig -OReleaseFast

Observed machine code:

0000000000000000 <f>:
   0:   55                      push   rbp
   1:   48 89 e5                mov    rbp,rsp
   4:   48 83 ec 08             sub    rsp,0x8
   8:   48 89 3c 24             mov    QWORD PTR [rsp],rdi
   c:   48 83 c7 01             add    rdi,0x1
  10:   48 89 f8                mov    rax,rdi
  13:   48 89 ec                mov    rsp,rbp
  16:   5d                      pop    rbp
  17:   c3                      ret

Expected machine code, something more like this:

f:
        lea     rax, [rdi + 1]
        ret

[jacobly0]

If the proposed transformation were split up into smaller steps, some of them might come into scope earlier, but as is, there are a lot of unrelated things being suggested here.