Use transform function on llvm::Module in the ExecutionEngine

The refactoring of ExecutionEngine dropped the usage of the irTransform function used to pass -O3 and other options to LLVM. As a consequence, the proper optimizations do not kick in in LLMV-land.

This CL makes use of the transform function and allows producing avx512 instructions, on an internal example, when using:
`mlir-cpu-runner -dump-object-file=1 -object-filename=foo.o` combined with `objdump -D foo.o`.

Assembly produced resembles:
```
    2b2e:       62 72 7d 48 18 04 0e    vbroadcastss (%rsi,%rcx,1),%zmm8
    2b35:       62 71 7c 48 28 ce       vmovaps %zmm6,%zmm9
    2b3b:       62 72 3d 48 a8 c9       vfmadd213ps %zmm1,%zmm8,%zmm9
    2b41:       62 f1 7c 48 28 cf       vmovaps %zmm7,%zmm1
    2b47:       62 f2 3d 48 a8 c8       vfmadd213ps %zmm0,%zmm8,%zmm1
    2b4d:       62 f2 7d 48 18 44 0e    vbroadcastss 0x4(%rsi,%rcx,1),%zmm0
    2b54:       01
    2b55:       62 71 7c 48 28 c6       vmovaps %zmm6,%zmm8
    2b5b:       62 72 7d 48 a8 c3       vfmadd213ps %zmm3,%zmm0,%zmm8
    2b61:       62 f1 7c 48 28 df       vmovaps %zmm7,%zmm3
    2b67:       62 f2 7d 48 a8 da       vfmadd213ps %zmm2,%zmm0,%zmm3
    2b6d:       62 f2 7d 48 18 44 0e    vbroadcastss 0x8(%rsi,%rcx,1),%zmm0
    2b74:       02
    2b75:       62 f2 7d 48 a8 f5       vfmadd213ps %zmm5,%zmm0,%zmm6
    2b7b:       62 f2 7d 48 a8 fc       vfmadd213ps %zmm4,%zmm0,%zmm7
```
etc.

Fixes #120

PiperOrigin-RevId: 267281097

Author: nicolasvasilache

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -277,6 +277,9 @@ Expected<std::unique_ptr<ExecutionEngine>> ExecutionEngine::create(
 
   // Add a ThreadSafemodule to the engine and return.
   ThreadSafeModule tsm(std::move(deserModule), std::move(ctx));
+  if (transformer)
+    cantFail(tsm.withModuleDo(
+        [&](llvm::Module &module) { return transformer(&module); }));
   cantFail(jit->addIRModule(std::move(tsm)));
   engine->jit = std::move(jit);