Do not override register storing MethodType in invokeExact.

As it might be in a callee-saved register and also known by GC
to have a reference.

arm64 and riscv are fine as they load MethodHandle's field into
a temp register and unpoison that register and not call site one.

Bug: 404465902

Test: ART_HEAP_POISONING=true art/test/testrunner/testrunner.py -b --host --optimizing --64
Test: ART_HEAP_POISONING=true art/test/testrunner/testrunner.py -b --host --optimizing --gcstress --64
Test: art/test/testrunner/testrunner.py -b --host --optimizing --64
Test: art/test/testrunner/testrunner.py -b --host --optimizing --gcstress --64

Change-Id: Ib1e80ee106a30b282f6bf2725c460f9c24d20c0e