x86 NCG SIMD: Support pack/insert/broadcast/unpack of 128-bit integer vectors

compiler/GHC/CmmToAsm/X86/CodeGen.hs

compiler/GHC/CmmToAsm/X86/Instr.hs

compiler/GHC/CmmToAsm/X86/Ppr.hs

testsuite/tests/simd/should_run/all.T

testsuite/tests/simd/should_run/int16x8_basic.hs

testsuite/tests/simd/should_run/int16x8_basic.stdout

testsuite/tests/simd/should_run/int16x8_basic_baseline.hs

...suite/tests/simd/should_run/int16x8_basic_baseline.stdout

testsuite/tests/simd/should_run/int32x4_basic.hs

testsuite/tests/simd/should_run/int32x4_basic.stdout

testsuite/tests/simd/should_run/int32x4_basic_baseline.hs

...suite/tests/simd/should_run/int32x4_basic_baseline.stdout

testsuite/tests/simd/should_run/int64x2_basic.hs

testsuite/tests/simd/should_run/int64x2_basic.stdout

testsuite/tests/simd/should_run/int64x2_basic_baseline.hs

...suite/tests/simd/should_run/int64x2_basic_baseline.stdout

testsuite/tests/simd/should_run/int8x16_basic.hs

testsuite/tests/simd/should_run/int8x16_basic.stdout

testsuite/tests/simd/should_run/int8x16_basic_baseline.hs

...suite/tests/simd/should_run/int8x16_basic_baseline.stdout