X86: Introduce the "relocImm" ComplexPattern, which represents a relocatable immediate.

A relocatable immediate is either an immediate operand or an operand that
can be relocated by the linker to an immediate, such as a regular symbol
in non-PIC code.

Start using relocImm for 32-bit and 64-bit MOV instructions, and for operands
of type "imm32_su". Remove a number of now-redundant patterns.

Differential Revision: https://reviews.llvm.org/D25812

llvm-svn: 286384