ebx/rbx only needs to be saved when 64-bit registers are supported
anyway. It should be fine to save/restore the whole rbx register
even in gnux32 where the base is technically just ebx.
This matches what we do for cmpxchg16b where rbx is saved/restored
regardless of gnux32.
pointer.
mwaitx uses EBX as one of its argument.
Using this instruction clobbers RBX as it is defined to hold one of the
input. When the backend uses dynamically allocated stack, RBX is used as
a reserved register for the base pointer.
This patch is adapted from @qcolombet patch for cmpxchg at r263325.
This fixes PR43528.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D73475
