Switch x86 SIMD bor from ORPS to POR encoding

There are two reasons for this change:
 1. it reduces confusion; using the `POR` encoding will match the future encodings of `band` and `bxor` and the `ORPS` encoding may be confusing as it is intended for floating-point operations
 2. `POR` has slightly more throughput: it only has to wait 0.33 cycles to execute again on all Intel architectures above Core whereas `ORPS` must wait 1 cycle on architectures older than Skylake (Intel Optimization Reference Manual, C.3)

`POR` does add one additional byte to the encoding and requires SSE2 so the `ORPS` opcode is left in for future use.

cranelift/codegen/meta/src/isa/x86/opcodes.rs

@@ -307,6 +307,9 @@ pub static POP_REG: [u8; 1] = [0x58];
 /// Returns the count of number of bits set to 1.
 pub static POPCNT: [u8; 3] = [0xf3, 0x0f, 0xb8];
 
+/// Bitwise OR of xmm2/m128 and xmm1 (SSE2).
+pub static POR: [u8; 3] = [0x66, 0x0f, 0xeb];
+
 /// Shuffle bytes in xmm1 according to contents of xmm2/m128 (SSE3).
 pub static PSHUFB: [u8; 4] = [0x66, 0x0f, 0x38, 0x00];