MacroAssemblerX86 should be happy with shift(cx, cx)

MacroAssemblerX86 should be happy with shift(cx, cx)
https://bugs.webkit.org/show_bug.cgi?id=154124

Reviewed by Saam Barati.

Prior to this change the assembler asserted that shift_amount and dest cannot be the same.
That's a good assertion for when shift_amount is not in cx. But if it's in cx already then
it's OK for them to be the same. Air will sometimes do shift(cx, cx) if you do "x << x" and
the coalescing got particularly clever.

* assembler/MacroAssemblerX86Common.h:
(JSC::MacroAssemblerX86Common::lshift32):
(JSC::MacroAssemblerX86Common::rshift32):
(JSC::MacroAssemblerX86Common::urshift32):
* assembler/MacroAssemblerX86_64.h:
(JSC::MacroAssemblerX86_64::lshift64):
(JSC::MacroAssemblerX86_64::rshift64):
(JSC::MacroAssemblerX86_64::urshift64):
* b3/testb3.cpp:
(JSC::B3::testLShiftSelf32):
(JSC::B3::testRShiftSelf32):
(JSC::B3::testURShiftSelf32):
(JSC::B3::testLShiftSelf64):
(JSC::B3::testRShiftSelf64):
(JSC::B3::testURShiftSelf64):
(JSC::B3::run):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@196433 268f45cc-cd09-0410-ab3c-d52691b4dbfc