PPC: [codegen] Change a few DCHECKs to V8_ASSUMEs

Port e5524920

Original Commit Message:

    This might or might not give clang-tidy a hint that the reported case
    (see issue) cannot happen. It might also generate slightly better code
    by giving hints to the compiler.
    Note that V8_ASSUME is actually a DCHECK in DEBUG builds, so we do not
    loose any checks here.

    Some DCHECKs were removed because they are redundant
    (RegisterBase::code() assumes to be only called on valid registers).

R=clemensb@chromium.org, joransiu@ca.ibm.com, junyan@redhat.com, midawson@redhat.com
BUG=
LOG=N

Change-Id: I653d398eb4b6b10fa769de62a9900edda95dd5db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3820583
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Reviewed-by: Junliang Yan <junyan@redhat.com>
Cr-Commit-Position: refs/heads/main@{#82318}

src/codegen/ppc/register-ppc.h

@@ -214,8 +214,9 @@ class DoubleRegister : public RegisterBase<DoubleRegister, kDoubleAfterLast> {
   // More details can be found here: https://crrev.com/c/2718472 . This is a
   // helper function to cast a Double to a Simdi128 register.
   Simd128Register toSimd() const {
-    DCHECK(base::IsInRange(static_cast<int>(code()), 0, kSimd128AfterLast - 1));
-    return Simd128Register(code());
+    int reg_code = code();
+    V8_ASSUME(reg_code >= 0 && reg_code < kSimd128AfterLast);
+    return Simd128Register(reg_code);
   }
 
  private: