X86: Stop LEA64_32r doing unspeakable things to its arguments.

Previously LEA64_32r went through virtually the entire backend thinking it was
using 32-bit registers until its blissful illusions were cruelly snatched away
by MCInstLower and 64-bit equivalents were substituted at the last minute.

This patch makes it behave normally, and take 64-bit registers as sources all
the way through. Previous uses (for 32-bit arithmetic) are accommodated via
SUBREG_TO_REG instructions which make the types and classes agree properly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183693 91177308-0d34-0410-b5e6-96231b3b80d8