Fix for PR7193 was overly conservative. The only case where sibcall callee

Fix for PR7193 was overly conservative. The only case where sibcall callee
address cannot be allocated a register is in 32-bit mode where the first
three arguments are marked inreg. In that case EAX, EDX, and ECX will be
used for argument passing.

This fixes PR7610.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108327 91177308-0d34-0410-b5e6-96231b3b80d8