Simplify intrinsic trimming

[Turnerj]

Performance is up to 15% faster but the main benefit is simplified codegen as that will have a bigger impact on the overall calculation.

Before

Method	Job	Runtime	TestString	Mean	Error	StdDev	Op/s	Code Size	Allocated
ForwardsTrim	Framework	.NET Framework 4.7.2	abcdefghij	6.938 ns	0.1753 ns	0.4133 ns	144,131,605.1	71 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcdefghij	9.927 ns	0.2467 ns	0.5816 ns	100,730,618.4	189 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcdefghij	10.022 ns	0.3716 ns	1.0359 ns	99,781,449.5	88 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcdefghij	10.249 ns	0.2463 ns	0.4627 ns	97,570,005.8	214 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [400]	19.667 ns	0.3660 ns	0.9513 ns	50,846,373.2	189 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [400]	26.456 ns	0.5387 ns	0.8699 ns	37,799,306.6	214 B	-
ForwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [400]	192.588 ns	3.8581 ns	5.1505 ns	5,192,422.0	71 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [400]	258.642 ns	4.7046 ns	4.1705 ns	3,866,341.5	88 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [8000]	379.279 ns	7.4101 ns	6.5689 ns	2,636,581.4	189 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [8000]	512.274 ns	10.0105 ns	19.0460 ns	1,952,080.5	214 B	-
ForwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [8000]	3,738.638 ns	73.9455 ns	117.2855 ns	267,477.1	71 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [8000]	5,021.090 ns	98.1290 ns	96.3758 ns	199,159.9	88 B	-

After

Method	Job	Runtime	TestString	Mean	Error	StdDev	Median	Op/s	Code Size	Allocated
ForwardsTrim	Framework	.NET Framework 4.7.2	abcdefghij	6.184 ns	0.1438 ns	0.1201 ns	6.217 ns	161,695,239.3	69 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcdefghij	6.531 ns	0.1138 ns	0.0889 ns	6.515 ns	153,107,920.4	136 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcdefghij	9.228 ns	0.1155 ns	0.1024 ns	9.190 ns	108,365,437.6	88 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcdefghij	10.114 ns	0.1722 ns	0.1438 ns	10.130 ns	98,873,604.3	189 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [400]	18.727 ns	0.4055 ns	0.6069 ns	18.757 ns	53,399,469.7	136 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [400]	22.687 ns	0.4718 ns	0.5616 ns	22.884 ns	44,077,596.8	189 B	-
ForwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [400]	166.039 ns	2.3966 ns	2.2418 ns	165.077 ns	6,022,681.8	69 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [400]	262.489 ns	5.1966 ns	6.9373 ns	261.323 ns	3,809,680.2	88 B	-
ForwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [8000]	319.906 ns	6.1976 ns	6.0868 ns	320.138 ns	3,125,918.0	136 B	-
BackwardsTrim	Core (All Intrinsics)	.NET 6.0	abcd(...)ghij [8000]	584.179 ns	49.5411 ns	146.0731 ns	523.157 ns	1,711,804.8	189 B	-
ForwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [8000]	3,990.212 ns	72.1029 ns	67.4451 ns	4,000.487 ns	250,613.2	69 B	-
BackwardsTrim	Framework	.NET Framework 4.7.2	abcd(...)ghij [8000]	6,005.445 ns	119.2689 ns	297.0209 ns	6,039.676 ns	166,515.6	88 B	-